SSL Laurent_Convergence.thy

theory Laurent_Convergence
  imports "HOL-Computational_Algebra.Formal_Laurent_Series" "HOL-Library.Landau_Symbols"
          Residue_Theorem

begin


definition%important fls_conv_radius :: "complex fls ==> ereal" where
  "fls_conv_radius f = fps_conv_radius (fls_regpart f)"

definition%important eval_fls :: "complex fls ==> complex ==> complex" where
  "eval_fls F z = eval_fps (fls_base_factor_to_fps F) z * z powi fls_subdegree F"

definition🍋‹tag important›
  has_laurent_expansion :: "(complex ==> complex) ==> complex fls ==> bool"
  (infixl ‹has'_laurent'_expansion› 60)
  where "(f has_laurent_expansion F) ⟷
            fls_conv_radius F > 0 ∧ eventually (λz. eval_fls F z = f z) (at 0)"

lemma has_laurent_expansion_schematicI:
  "f has_laurent_expansion F ==> F = G ==> f has_laurent_expansion G"
  by simp

lemma has_laurent_expansion_cong:
  assumes "eventually (λx. f x = g x) (at 0)" "F = G"
  shows   "(f has_laurent_expansion F) ⟷ (g has_laurent_expansion G)"
proof -
  have "eventually (λz. eval_fls F z = g z) (at 0)"
    if "eventually (λz. eval_fls F z = f z) (at 0)" "eventually (λx. f x = g x) (at 0)" for f g
    using that by eventually_elim auto
  from this[of f g] this[of g f] show ?thesis
    using assms by (auto simp: eq_commute has_laurent_expansion_def)
qed

lemma has_laurent_expansion_cong':
  assumes "eventually (λx. f x = g x) (at z)" "F = G" "z = z'"
  shows   "((λx. f (z + x)) has_laurent_expansion F) ⟷ ((λx. g (z' + x)) has_laurent_expansion G)"
  by (intro has_laurent_expansion_cong)
     (use assms in ‹auto simp: at_to_0' eventually_filtermap add_ac›)

lemma fls_conv_radius_altdef:
  "fls_conv_radius F = fps_conv_radius (fls_base_factor_to_fps F)"
proof -
  have "conv_radius (λn. fls_nth F (int n)) = conv_radius (λn. fls_nth F (int n + fls_subdegree F))"
  proof (cases "fls_subdegree F ≥ 0")
    case True
    hence "conv_radius (λn. fls_nth F (int n + fls_subdegree F)) =
           conv_radius (λn. fls_nth F (int (n + nat (fls_subdegree F))))"
      by auto
    thus ?thesis
      by (subst (asm) conv_radius_shift) auto
  next
    case False
    hence "conv_radius (λn. fls_nth F (int n)) =
           conv_radius (λn. fls_nth F (fls_subdegree F + int (n + nat (-fls_subdegree F))))"
      by auto
    thus ?thesis
      by (subst (asm) conv_radius_shift) (auto simp: add_ac)
  qed
  thus ?thesis
    by (simp add: fls_conv_radius_def fps_conv_radius_def)
qed

lemma eval_fps_of_nat [simp]: "eval_fps (of_nat n) z = of_nat n"
  and eval_fps_of_int [simp]: "eval_fps (of_int m) z = of_int m"
  by (simp_all flip: fps_of_nat fps_of_int)

lemma fps_conv_radius_of_nat [simp]: "fps_conv_radius (of_nat n) = ∞"
  and fps_conv_radius_of_int [simp]: "fps_conv_radius (of_int m) = ∞"
  by (simp_all flip: fps_of_nat fps_of_int)

lemma fps_conv_radius_fls_regpart: "fps_conv_radius (fls_regpart F) = fls_conv_radius F"
  by (simp add: fls_conv_radius_def)

lemma fls_conv_radius_0 [simp]: "fls_conv_radius 0 = ∞"
  and fls_conv_radius_1 [simp]: "fls_conv_radius 1 = ∞"
  and fls_conv_radius_const [simp]: "fls_conv_radius (fls_const c) = ∞"
  and fls_conv_radius_numeral [simp]: "fls_conv_radius (numeral num) = ∞"
  and fls_conv_radius_of_nat [simp]: "fls_conv_radius (of_nat n) = ∞"
  and fls_conv_radius_of_int [simp]: "fls_conv_radius (of_int m) = ∞"
  and fls_conv_radius_X [simp]: "fls_conv_radius fls_X = ∞"
  and fls_conv_radius_X_inv [simp]: "fls_conv_radius fls_X_inv = ∞"
  and fls_conv_radius_X_intpow [simp]: "fls_conv_radius (fls_X_intpow m) = ∞"
  by (simp_all add: fls_conv_radius_def fls_X_intpow_regpart)

lemma fls_conv_radius_shift [simp]: "fls_conv_radius (fls_shift n F) = fls_conv_radius F"
  unfolding fls_conv_radius_altdef by (subst fls_base_factor_to_fps_shift) (rule refl)

lemma fls_conv_radius_fps_to_fls [simp]: "fls_conv_radius (fps_to_fls F) = fps_conv_radius F"
  by (simp add: fls_conv_radius_def)

lemma fls_conv_radius_deriv [simp]: "fls_conv_radius (fls_deriv F) ≥ fls_conv_radius F"
proof -
  have "fls_conv_radius (fls_deriv F) = fps_conv_radius (fls_regpart (fls_deriv F))"
    by (simp add: fls_conv_radius_def)
  also have "fls_regpart (fls_deriv F) = fps_deriv (fls_regpart F)"
    by (intro fps_ext) (auto simp: add_ac)
  also have "fps_conv_radius … ≥ fls_conv_radius F"
    by (simp add: fls_conv_radius_def fps_conv_radius_deriv)
  finally show ?thesis .
qed

lemma fls_conv_radius_uminus [simp]: "fls_conv_radius (-F) = fls_conv_radius F"
  by (simp add: fls_conv_radius_def)

lemma fls_conv_radius_add: "fls_conv_radius (F + G) ≥ min (fls_conv_radius F) (fls_conv_radius G)"
  by (simp add: fls_conv_radius_def fps_conv_radius_add)

lemma fls_conv_radius_diff: "fls_conv_radius (F - G) ≥ min (fls_conv_radius F) (fls_conv_radius G)"
  by (simp add: fls_conv_radius_def fps_conv_radius_diff)

lemma fls_conv_radius_mult: "fls_conv_radius (F * G) ≥ min (fls_conv_radius F) (fls_conv_radius G)"
proof (cases "F = 0 ∨ G = 0")
  case False
  hence [simp]: "F ≠ 0" "G ≠ 0"
    by auto
  have "fls_conv_radius (F * G) = fps_conv_radius (fls_regpart (fls_shift (fls_subdegree F + fls_subdegree G) (F * G)))"
    by (simp add: fls_conv_radius_altdef)
  also have "fls_regpart (fls_shift (fls_subdegree F + fls_subdegree G) (F * G)) =
             fls_base_factor_to_fps F * fls_base_factor_to_fps G"
    by (simp add: fls_times_def)
  also have "fps_conv_radius … ≥ min (fls_conv_radius F) (fls_conv_radius G)"
    unfolding fls_conv_radius_altdef by (rule fps_conv_radius_mult)
  finally show ?thesis .
qed auto

lemma fps_conv_radius_add_ge:
  "fps_conv_radius F ≥ r ==> fps_conv_radius G ≥ r ==> fps_conv_radius (F + G) ≥ r"
  using fps_conv_radius_add[of F G] by (simp add: min_def split: if_splits)

lemma fps_conv_radius_diff_ge:
  "fps_conv_radius F ≥ r ==> fps_conv_radius G ≥ r ==> fps_conv_radius (F - G) ≥ r"
  using fps_conv_radius_diff[of F G] by (simp add: min_def split: if_splits)

lemma fps_conv_radius_mult_ge:
  "fps_conv_radius F ≥ r ==> fps_conv_radius G ≥ r ==> fps_conv_radius (F * G) ≥ r"
  using fps_conv_radius_mult[of F G] by (simp add: min_def split: if_splits)

lemma fls_conv_radius_add_ge:
  "fls_conv_radius F ≥ r ==> fls_conv_radius G ≥ r ==> fls_conv_radius (F + G) ≥ r"
  using fls_conv_radius_add[of F G] by (simp add: min_def split: if_splits)

lemma fls_conv_radius_diff_ge:
  "fls_conv_radius F ≥ r ==> fls_conv_radius G ≥ r ==> fls_conv_radius (F - G) ≥ r"
  using fls_conv_radius_diff[of F G] by (simp add: min_def split: if_splits)

lemma fls_conv_radius_mult_ge:
  "fls_conv_radius F ≥ r ==> fls_conv_radius G ≥ r ==> fls_conv_radius (F * G) ≥ r"
  using fls_conv_radius_mult[of F G] by (simp add: min_def split: if_splits)

lemma fls_conv_radius_power: "fls_conv_radius (F ^ n) ≥ fls_conv_radius F"
  by (induction n) (auto intro!: fls_conv_radius_mult_ge)

lemma eval_fls_0 [simp]: "eval_fls 0 z = 0"
  and eval_fls_1 [simp]: "eval_fls 1 z = 1"
  and eval_fls_const [simp]: "eval_fls (fls_const c) z = c"
  and eval_fls_numeral [simp]: "eval_fls (numeral num) z = numeral num"
  and eval_fls_of_nat [simp]: "eval_fls (of_nat n) z = of_nat n"
  and eval_fls_of_int [simp]: "eval_fls (of_int m) z = of_int m"
  and eval_fls_X [simp]: "eval_fls fls_X z = z"
  and eval_fls_X_intpow [simp]: "eval_fls (fls_X_intpow m) z = z powi m"
  by (simp_all add: eval_fls_def)

lemma eval_fls_at_0: "eval_fls F 0 = (if fls_subdegree F ≥ 0 then fls_nth F 0 else 0)"
  by (cases "fls_subdegree F = 0")
     (simp_all add: eval_fls_def fls_regpart_def eval_fps_at_0)

lemma eval_fps_to_fls:
  assumes "norm z < fps_conv_radius F"
  shows   "eval_fls (fps_to_fls F) z = eval_fps F z"
proof (cases "F = 0")
  case [simp]: False
  have "eval_fps F z = eval_fps (unit_factor F * normalize F) z"
    by (metis unit_factor_mult_normalize)
  also have "… = eval_fps (unit_factor F * fps_X ^ subdegree F) z"
    by simp
  also have "… = eval_fps (unit_factor F) z * z ^ subdegree F"
    using assms by (subst eval_fps_mult) auto
  also have "… = eval_fls (fps_to_fls F) z"
    unfolding eval_fls_def fls_base_factor_to_fps_to_fls fls_subdegree_fls_to_fps
              power_int_of_nat ..
  finally show ?thesis ..
qed auto

lemma eval_fls_shift:
  assumes [simp]: "z ≠ 0"
  shows   "eval_fls (fls_shift n F) z = eval_fls F z * z powi -n"
proof (cases "F = 0")
  case [simp]: False
  show ?thesis
  unfolding eval_fls_def
  by (subst fls_base_factor_to_fps_shift, subst fls_shift_subdegree[OF ‹F ≠ 0›], subst power_int_diff)
     (auto simp: power_int_minus divide_simps)
qed auto

lemma eval_fls_add:
  assumes "ereal (norm z) < fls_conv_radius F" "ereal (norm z) < fls_conv_radius G" "z ≠ 0"
  shows   "eval_fls (F + G) z = eval_fls F z + eval_fls G z"
  using assms
proof (induction "fls_subdegree F" "fls_subdegree G" arbitrary: F G rule: linorder_wlog)
  case (sym F G)
  show ?case
    using sym(1)[of G F] sym(2-) by (simp add: add_ac)
next
  case (le F G)
  show ?case
  proof (cases "F = 0 ∨ G = 0")
    case False
    hence [simp]: "F ≠ 0" "G ≠ 0"
      by auto
    note [simp] = ‹z ≠ 0›
    define F' G' where "F' = fls_base_factor_to_fps F" "G' = fls_base_factor_to_fps G"
    define m n where "m = fls_subdegree F" "n = fls_subdegree G"
    have "m ≤ n"
      using le by (auto simp: m_n_def)
    have conv1: "ereal (cmod z) < fps_conv_radius F'" "ereal (cmod z) < fps_conv_radius G'"
      using assms le by (simp_all add: F'_G'_def fls_conv_radius_altdef)
    have conv2: "ereal (cmod z) < fps_conv_radius (G' * fps_X ^ nat (n - m))"
      using conv1 by (intro less_le_trans[OF _ fps_conv_radius_mult]) auto
    have conv3: "ereal (cmod z) < fps_conv_radius (F' + G' * fps_X ^ nat (n - m))"
      using conv1 conv2 by (intro less_le_trans[OF _ fps_conv_radius_add]) auto

    have "eval_fls F z + eval_fls G z = eval_fps F' z * z powi m + eval_fps G' z * z powi n"
      unfolding eval_fls_def m_n_def[symmetric] F'_G'_def[symmetric]
      by (simp add: power_int_add algebra_simps)
    also have "… = (eval_fps F' z + eval_fps G' z * z powi (n - m)) * z powi m"
      by (simp add: algebra_simps power_int_diff)
    also have "eval_fps G' z * z powi (n - m) = eval_fps (G' * fps_X ^ nat (n - m)) z"
      using assms ‹m ≤ n› conv1 by (subst eval_fps_mult) (auto simp: power_int_def)
    also have "eval_fps F' z + … = eval_fps (F' + G' * fps_X ^ nat (n - m)) z"
      using conv1 conv2 by (subst eval_fps_add) auto
    also have "… = eval_fls (fps_to_fls (F' + G' * fps_X ^ nat (n - m))) z"
      using conv3 by (subst eval_fps_to_fls) auto
    also have "… * z powi m = eval_fls (fls_shift (-m) (fps_to_fls (F' + G' * fps_X ^ nat (n - m)))) z"
      by (subst eval_fls_shift) auto
    also have "fls_shift (-m) (fps_to_fls (F' + G' * fps_X ^ nat (n - m))) = F + G"
      using ‹m ≤ n›
      by (simp add: fls_times_fps_to_fls fps_to_fls_power fls_X_power_conv_shift_1
                    fls_shifted_times_simps F'_G'_def m_n_def)
    finally show ?thesis ..
  qed auto
qed

lemma eval_fls_minus:
  assumes "ereal (norm z) < fls_conv_radius F"
  shows   "eval_fls (-F) z = -eval_fls F z"
  using assms by (simp add: eval_fls_def eval_fps_minus fls_conv_radius_altdef)

lemma eval_fls_diff:
  assumes "ereal (norm z) < fls_conv_radius F" "ereal (norm z) < fls_conv_radius G"
     and [simp]: "z ≠ 0"
  shows   "eval_fls (F - G) z = eval_fls F z - eval_fls G z"
proof -
  have "eval_fls (F + (-G)) z = eval_fls F z - eval_fls G z"
    using assms by (subst eval_fls_add) (auto simp: eval_fls_minus)
  thus ?thesis
    by simp
qed

lemma eval_fls_mult:
  assumes "ereal (norm z) < fls_conv_radius F" "ereal (norm z) < fls_conv_radius G" "z ≠ 0"
  shows   "eval_fls (F * G) z = eval_fls F z * eval_fls G z"
proof (cases "F = 0 ∨ G = 0")
  case False
  hence [simp]: "F ≠ 0" "G ≠ 0"
    by auto
  note [simp] = ‹z ≠ 0›
  define F' G' where "F' = fls_base_factor_to_fps F" "G' = fls_base_factor_to_fps G"
  define m n where "m = fls_subdegree F" "n = fls_subdegree G"
  have "eval_fls F z * eval_fls G z = (eval_fps F' z * eval_fps G' z) * z powi (m + n)"
    unfolding eval_fls_def m_n_def[symmetric] F'_G'_def[symmetric]
    by (simp add: power_int_add algebra_simps)
  also have "… = eval_fps (F' * G') z * z powi (m + n)"
    using assms by (subst eval_fps_mult) (auto simp: F'_G'_def fls_conv_radius_altdef)
  also have "… = eval_fls (F * G) z"
    by (simp add: eval_fls_def F'_G'_def m_n_def) (simp add: fls_times_def)
  finally show ?thesis ..
qed auto

lemma eval_fls_power:
  assumes "ereal (norm z) < fls_conv_radius F" "z ≠ 0"
  shows   "eval_fls (F ^ n) z = eval_fls F z ^ n"
proof (induction n)
  case (Suc n)
  have "eval_fls (F ^ Suc n) z = eval_fls (F * F ^ n) z"
    by simp
  also have "… = eval_fls F z * eval_fls (F ^ n) z"
    using assms by (subst eval_fls_mult) (auto intro!: less_le_trans[OF _ fls_conv_radius_power])
  finally show ?case
    using Suc by simp
qed auto

lemma eval_fls_eq:
  assumes "N ≤ fls_subdegree F" "fls_subdegree F ≥ 0 ∨ z ≠ 0"
  assumes "(λn. fls_nth F (int n + N) * z powi (int n + N)) sums S"
  shows   "eval_fls F z = S"
proof (cases "z = 0")
  case [simp]: True
  have "(λn. fls_nth F (int n + N) * z powi (int n + N)) =
        (λn. if n ∈ (if N ≤ 0 then {nat (-N)} else {}) then fls_nth F (int n + N) else 0)"
    by (auto simp: fun_eq_iff split: if_splits)
  also have "… sums (∑n∈(if N ≤ 0 then {nat (-N)} else {}). fls_nth F (int n + N))"
    by (rule sums_If_finite_set) auto
  also have "… = fls_nth F 0"
    using assms by auto
  also have "… = eval_fls F z"
    using assms by (auto simp: eval_fls_def eval_fps_at_0 power_int_0_left_if)
  finally show ?thesis 
    using assms by (simp add: sums_iff)
next
  case [simp]: False
  define N' where "N' = fls_subdegree F"
  define d where "d = nat (N' - N)"

  have "(λn. fls_nth F (int n + N) * z powi (int n + N)) sums S"
    by fact
  also have "?this ⟷ (λn. fls_nth F (int (n+d) + N) * z powi (int (n+d) + N)) sums S"
    by (rule sums_zero_iff_shift [symmetric]) (use assms in ‹auto simp: d_def N'_def›)
  also have "(λn. int (n+d) + N) = (λn. int n + N')"
    using assms by (auto simp: N'_def d_def)
  finally have "(λn. fls_nth F (int n + N') * z powi (int n + N')) sums S" .
  hence "(λn. z powi (-N') * (fls_nth F (int n + N') * z powi (int n + N'))) sums (z powi (-N') * S)"
    by (intro sums_mult)
  hence "(λn. fls_nth F (int n + N') * z ^ n) sums (z powi (-N') * S)"
    by (simp add: power_int_add power_int_minus field_simps)
  thus ?thesis
    by (simp add: eval_fls_def eval_fps_def sums_iff power_int_minus N'_def)
qed

lemma norm_summable_fls:
  "norm z < fls_conv_radius f ==> summable (λn. norm (fls_nth f n * z ^ n))"
  using norm_summable_fps[of z "fls_regpart f"] by (simp add: fls_conv_radius_def)

lemma norm_summable_fls':
  "norm z < fls_conv_radius f ==> summable (λn. norm (fls_nth f (n + fls_subdegree f) * z ^ n))"
  using norm_summable_fps[of z "fls_base_factor_to_fps f"] by (simp add: fls_conv_radius_altdef)

lemma summable_fls:
  "norm z < fls_conv_radius f ==> summable (λn. fls_nth f n * z ^ n)"
  by (rule summable_norm_cancel[OF norm_summable_fls])

theorem sums_eval_fls:
  fixes f
  defines "n ≡ fls_subdegree f"
  assumes "norm z < fls_conv_radius f" and "z ≠ 0 ∨ n ≥ 0"
  shows   "(λk. fls_nth f (int k + n) * z powi (int k + n)) sums eval_fls f z"
proof (cases "z = 0")
  case [simp]: False
  have "(λk. fps_nth (fls_base_factor_to_fps f) k * z ^ k * z powi n) sums
          (eval_fps (fls_base_factor_to_fps f) z * z powi n)"
    using assms(2) by (intro sums_eval_fps sums_mult2) (auto simp: fls_conv_radius_altdef)
  thus ?thesis
    by (simp add: power_int_add n_def eval_fls_def mult_ac)
next
  case [simp]: True
  with assms have "n ≥ 0"
    by auto
  have "(λk. fls_nth f (int k + n) * z powi (int k + n)) sums
          (∑k∈(if n ≤ 0 then {nat (-n)} else {}). fls_nth f (int k + n) * z powi (int k + n))"
    by (intro sums_finite) (auto split: if_splits)
  also have "… = eval_fls f z"
    using ‹n ≥ 0› by (auto simp: eval_fls_at_0 n_def not_le)
  finally show ?thesis .
qed

lemma holomorphic_on_eval_fls:
  fixes f
  defines "n ≡ fls_subdegree f"
  assumes "A ⊆ eball 0 (fls_conv_radius f) - (if n ≥ 0 then {} else {0})"
  shows   "eval_fls f holomorphic_on A"
proof (cases "n ≥ 0")
  case True
  have "eval_fls f = (λz. eval_fps (fls_base_factor_to_fps f) z * z ^ nat n)"
    using True by (simp add: fun_eq_iff eval_fls_def power_int_def n_def)
  moreover have "… holomorphic_on A"
    using True assms(2) by (intro holomorphic_intros) (auto simp: fls_conv_radius_altdef)
  ultimately show ?thesis
    by simp
next
  case False
  show ?thesis using assms
    unfolding eval_fls_def by (intro holomorphic_intros) (auto simp: fls_conv_radius_altdef)
qed

lemma holomorphic_on_eval_fls' [holomorphic_intros]:
  assumes "g holomorphic_on A"
  assumes "g ` A ⊆ eball 0 (fls_conv_radius f) - (if fls_subdegree f ≥ 0 then {} else {0})"
  shows   "(λx. eval_fls f (g x)) holomorphic_on A"
  by (meson assms holomorphic_on_compose holomorphic_on_eval_fls holomorphic_transform o_def)

lemma continuous_on_eval_fls:
  fixes f
  defines "n ≡ fls_subdegree f"
  assumes "A ⊆ eball 0 (fls_conv_radius f) - (if n ≥ 0 then {} else {0})"
  shows   "continuous_on A (eval_fls f)"
  using assms holomorphic_on_eval_fls holomorphic_on_imp_continuous_on by blast

lemma continuous_on_eval_fls' [continuous_intros]:
  fixes f
  defines "n ≡ fls_subdegree f"
  assumes "g ` A ⊆ eball 0 (fls_conv_radius f) - (if n ≥ 0 then {} else {0})"
  assumes "continuous_on A g"
  shows   "continuous_on A (λx. eval_fls f (g x))"
  by (metis assms continuous_on_compose2 continuous_on_eval_fls order.refl)

lemmas has_field_derivative_eval_fps' [derivative_intros] =
  DERIV_chain2[OF has_field_derivative_eval_fps]

(* TODO: generalise for nonneg subdegree *)
lemma has_field_derivative_eval_fls:
  assumes "z ∈ eball 0 (fls_conv_radius f) - {0}"
  shows   "(eval_fls f has_field_derivative eval_fls (fls_deriv f) z) (at z within A)"
proof -
  define g where "g = fls_base_factor_to_fps f"
  define n where "n = fls_subdegree f"
  have [simp]: "fps_conv_radius g = fls_conv_radius f"
    by (simp add: fls_conv_radius_altdef g_def)
  have conv1: "fps_conv_radius (fps_deriv g * fps_X) ≥ fls_conv_radius f"
    by (intro fps_conv_radius_mult_ge order.trans[OF _ fps_conv_radius_deriv]) auto
  have conv2: "fps_conv_radius (of_int n * g) ≥ fls_conv_radius f"
    by (intro fps_conv_radius_mult_ge) auto
  have conv3: "fps_conv_radius (fps_deriv g * fps_X + of_int n * g) ≥ fls_conv_radius f"
    by (intro fps_conv_radius_add_ge conv1 conv2)

  have [simp]: "fps_conv_radius g = fls_conv_radius f"
    by (simp add: g_def fls_conv_radius_altdef)
  have "((λz. eval_fps g z * z powi fls_subdegree f) has_field_derivative
          (eval_fps (fps_deriv g) z * z powi n + of_int n * z powi (n - 1) * eval_fps g z))
          (at z within A)"
    using assms by (auto intro!: derivative_eq_intros simp: n_def)
  also have "(λz. eval_fps g z * z powi fls_subdegree f) = eval_fls f"
    by (simp add: eval_fls_def g_def fun_eq_iff)
  also have "eval_fps (fps_deriv g) z * z powi n + of_int n * z powi (n - 1) * eval_fps g z =
             (z * eval_fps (fps_deriv g) z + of_int n * eval_fps g z) * z powi (n - 1)"
    using assms by (auto simp: power_int_diff field_simps)
  also have "(z * eval_fps (fps_deriv g) z + of_int n * eval_fps g z) =
             eval_fps (fps_deriv g * fps_X + of_int n * g) z"
    using conv1 conv2 assms fps_conv_radius_deriv[of g]
    by (subst eval_fps_add) (auto simp: eval_fps_mult)
  also have "… = eval_fls (fps_to_fls (fps_deriv g * fps_X + of_int n * g)) z"
    using conv3 assms by (subst eval_fps_to_fls) auto
  also have "… * z powi (n - 1) = eval_fls (fls_shift (1 - n) (fps_to_fls (fps_deriv g * fps_X + of_int n * g))) z"
    using assms by (subst eval_fls_shift) auto
  also have "fls_shift (1 - n) (fps_to_fls (fps_deriv g * fps_X + of_int n * g)) = fls_deriv f"
    by (intro fls_eqI) (auto simp: g_def n_def algebra_simps eq_commute[of _ "fls_subdegree f"])
  finally show ?thesis .
qed

lemma eval_fls_deriv:
  assumes "z ∈ eball 0 (fls_conv_radius F) - {0}"
  shows   "eval_fls (fls_deriv F) z = deriv (eval_fls F) z"
  by (metis DERIV_imp_deriv assms has_field_derivative_eval_fls)

lemma analytic_on_eval_fls:
  assumes "A ⊆ eball 0 (fls_conv_radius f) - (if fls_subdegree f ≥ 0 then {} else {0})"
  shows   "eval_fls f analytic_on A"
proof (rule analytic_on_subset [OF _ assms])
  show "eval_fls f analytic_on eball 0 (fls_conv_radius f) - (if fls_subdegree f ≥ 0 then {} else {0})"
    using holomorphic_on_eval_fls[OF order.refl]
    by (subst analytic_on_open) auto
qed

lemma analytic_on_eval_fls' [analytic_intros]:
  assumes "g analytic_on A"
  assumes "g ` A ⊆ eball 0 (fls_conv_radius f) - (if fls_subdegree f ≥ 0 then {} else {0})"
  shows   "(λx. eval_fls f (g x)) analytic_on A"
proof -
  have "eval_fls f ∘ g analytic_on A"
    by (intro analytic_on_compose[OF assms(1) analytic_on_eval_fls]) (use assms in auto)
  thus ?thesis
    by (simp add: o_def)
qed

lemma continuous_eval_fls [continuous_intros]:
  assumes "z ∈ eball 0 (fls_conv_radius F) - (if fls_subdegree F ≥ 0 then {} else {0})"
  shows   "continuous (at z within A) (eval_fls F)"
proof -
  have "isCont (eval_fls F) z"
    using continuous_on_eval_fls[OF order.refl] assms
    by (subst (asm) continuous_on_eq_continuous_at) auto
  thus ?thesis
    using continuous_at_imp_continuous_at_within by blast
qed




named_theorems laurent_expansion_intros

lemma has_laurent_expansion_imp_asymp_equiv_0:
  assumes F: "f has_laurent_expansion F"
  defines "n ≡ fls_subdegree F"
  shows   "f ∼[at 0] (λz. fls_nth F n * z powi n)"
proof (cases "F = 0")
  case True
  thus ?thesis using assms
    by (auto simp: has_laurent_expansion_def)
next
  case [simp]: False
  define G where "G = fls_base_factor_to_fps F"
  have "fls_conv_radius F > 0"
    using F by (auto simp: has_laurent_expansion_def)
  hence "isCont (eval_fps G) 0"
    by (intro continuous_intros) (auto simp: G_def fps_conv_radius_fls_regpart zero_ereal_def)
  hence lim: "eval_fps G ←-0→ eval_fps G 0"
    by (meson isContD)
  have [simp]: "fps_nth G 0 ≠ 0"
    by (auto simp: G_def)

  have "f ∼[at 0] eval_fls F"
    using F by (intro asymp_equiv_refl_ev) (auto simp: has_laurent_expansion_def eq_commute)
  also have "… = (λz. eval_fps G z * z powi n)"
    by (intro ext) (simp_all add: eval_fls_def G_def n_def)
  also have "… ∼[at 0] (λz. fps_nth G 0 * z powi n)" using lim
    by (intro asymp_equiv_intros tendsto_imp_asymp_equiv_const) (auto simp: eval_fps_at_0)
  also have "fps_nth G 0 = fls_nth F n"
    by (simp add: G_def n_def)
  finally show ?thesis
    by simp
qed

lemma has_laurent_expansion_imp_asymp_equiv:
  assumes F: "(λw. f (z + w)) has_laurent_expansion F"
  defines "n ≡ fls_subdegree F"
  shows   "f ∼[at z] (λw. fls_nth F n * (w - z) powi n)"
  using has_laurent_expansion_imp_asymp_equiv_0[OF assms(1)] unfolding n_def
  by (simp add: at_to_0[of z] asymp_equiv_filtermap_iff add_ac)


lemmas [tendsto_intros del] = tendsto_power_int

lemma has_laurent_expansion_imp_tendsto_0:
  assumes F: "f has_laurent_expansion F" and "fls_subdegree F ≥ 0"
  shows   "f ←-0→ fls_nth F 0"
proof (rule asymp_equiv_tendsto_transfer)
  show "(λz. fls_nth F (fls_subdegree F) * z powi fls_subdegree F) ∼[at 0] f"
    by (rule asymp_equiv_symI, rule has_laurent_expansion_imp_asymp_equiv_0) fact
  show "(λz. fls_nth F (fls_subdegree F) * z powi fls_subdegree F) ←-0→ fls_nth F 0"
    by (rule tendsto_eq_intros refl | use assms(2) in simp)+
       (use assms(2) in ‹auto simp: power_int_0_left_if›)
qed

lemma has_laurent_expansion_imp_tendsto:
  assumes F: "(λw. f (z + w)) has_laurent_expansion F" and "fls_subdegree F ≥ 0"
  shows   "f ←-z→ fls_nth F 0"
  using has_laurent_expansion_imp_tendsto_0[OF assms]
  by (simp add: at_to_0[of z] filterlim_filtermap add_ac)

lemma has_laurent_expansion_imp_filterlim_infinity_0:
  assumes F: "f has_laurent_expansion F" and "fls_subdegree F < 0"
  shows   "filterlim f at_infinity (at 0)"
proof (rule asymp_equiv_at_infinity_transfer)
  have [simp]: "F ≠ 0"
    using assms(2) by auto
  show "(λz. fls_nth F (fls_subdegree F) * z powi fls_subdegree F) ∼[at 0] f"
    by (rule asymp_equiv_symI, rule has_laurent_expansion_imp_asymp_equiv_0) fact
  show "filterlim (λz. fls_nth F (fls_subdegree F) * z powi fls_subdegree F) at_infinity (at 0)"
    by (rule tendsto_mult_filterlim_at_infinity tendsto_const
             filterlim_power_int_neg_at_infinity | use assms(2) in simp)+
       (auto simp: eventually_at_filter)
qed

lemma has_laurent_expansion_imp_neg_fls_subdegree:
  assumes F: "f has_laurent_expansion F"
    and infy:"filterlim f at_infinity (at 0)"
  shows   "fls_subdegree F < 0"
proof (rule ccontr)
  assume asm:"¬ fls_subdegree F < 0"
  define ff where "ff=(λz. fls_nth F (fls_subdegree F)
                              * z powi fls_subdegree F)"

  have "(ff ---> (if fls_subdegree F =0 then fls_nth F 0 else 0)) (at 0)"
    using asm unfolding ff_def
    by (auto intro!: tendsto_eq_intros)
  moreover have "filterlim ff at_infinity (at 0)"
  proof (rule asymp_equiv_at_infinity_transfer)
    show "f ∼[at 0] ff" unfolding ff_def
      using has_laurent_expansion_imp_asymp_equiv_0[OF F] unfolding ff_def .
    show "filterlim f at_infinity (at 0)" by fact
  qed
  ultimately show False
    using not_tendsto_and_filterlim_at_infinity[of "at (0::complex)"] by auto
qed

lemma has_laurent_expansion_imp_filterlim_infinity:
  assumes F: "(λw. f (z + w)) has_laurent_expansion F" and "fls_subdegree F < 0"
  shows   "filterlim f at_infinity (at z)"
  using has_laurent_expansion_imp_filterlim_infinity_0[OF assms]
  by (simp add: at_to_0[of z] filterlim_filtermap add_ac)

lemma has_laurent_expansion_imp_is_pole_0:
  assumes F: "f has_laurent_expansion F" and "fls_subdegree F < 0"
  shows   "is_pole f 0"
  using has_laurent_expansion_imp_filterlim_infinity_0[OF assms]
  by (simp add: is_pole_def)

lemma is_pole_0_imp_neg_fls_subdegree:
  assumes F: "f has_laurent_expansion F" and "is_pole f 0"
  shows   "fls_subdegree F < 0"
  using F assms(2) has_laurent_expansion_imp_neg_fls_subdegree is_pole_def
  by blast

lemma has_laurent_expansion_imp_is_pole:
  assumes F: "(λx. f (z + x)) has_laurent_expansion F" and "fls_subdegree F < 0"
  shows   "is_pole f z"
  using has_laurent_expansion_imp_is_pole_0[OF assms]
  by (simp add: is_pole_shift_0')

lemma is_pole_imp_neg_fls_subdegree:
  assumes F: "(λx. f (z + x)) has_laurent_expansion F" and "is_pole f z"
  shows   "fls_subdegree F < 0"
proof -
  have "is_pole (λx. f (z + x)) 0"
    using assms(2) is_pole_shift_0 by blast
  then show ?thesis
    using F is_pole_0_imp_neg_fls_subdegree by blast
qed

lemma is_pole_fls_subdegree_iff:
  assumes "(λx. f (z + x)) has_laurent_expansion F"
  shows "is_pole f z ⟷ fls_subdegree F < 0"
  using assms is_pole_imp_neg_fls_subdegree has_laurent_expansion_imp_is_pole
  by auto

lemma
  assumes "f has_laurent_expansion F"
  shows   has_laurent_expansion_isolated_0: "isolated_singularity_at f 0"
    and   has_laurent_expansion_not_essential_0: "not_essential f 0"
proof -
  from assms have "eventually (λz. eval_fls F z = f z) (at 0)"
    by (auto simp: has_laurent_expansion_def)
  then obtain r where r: "r > 0" "∧z. z ∈ ball 0 r - {0} ==> eval_fls F z = f z"
    by (auto simp: eventually_at_filter ball_def eventually_nhds_metric)

  have "fls_conv_radius F > 0"
    using assms by (auto simp: has_laurent_expansion_def)
  then obtain R :: real where R: "R > 0" "R ≤ min r (fls_conv_radius F)"
    using ‹r > 0› by (metis dual_order.strict_implies_order ereal_dense2 ereal_less(2) min_def)

  have "eval_fls F holomorphic_on ball 0 R - {0}"
    using r R by (intro holomorphic_intros ball_eball_mono Diff_mono)  (auto simp: ereal_le_less)
  also have "?this ⟷ f holomorphic_on ball 0 R - {0}"
    using r R by (intro holomorphic_cong) auto
  also have "… ⟷ f analytic_on ball 0 R - {0}"
    by (subst analytic_on_open) auto
  finally show "isolated_singularity_at f 0"
    unfolding isolated_singularity_at_def using ‹R > 0› by blast

  show "not_essential f 0"
  proof (cases "fls_subdegree F ≥ 0")
    case True
    hence "f ←-0→ fls_nth F 0"
      by (intro has_laurent_expansion_imp_tendsto_0[OF assms])
    thus ?thesis
      by (auto simp: not_essential_def)
  next
    case False
    hence "is_pole f 0"
      by (intro has_laurent_expansion_imp_is_pole_0[OF assms]) auto
    thus ?thesis
      by (auto simp: not_essential_def)
  qed
qed

lemma
  assumes "(λw. f (z + w)) has_laurent_expansion F"
  shows   has_laurent_expansion_isolated: "isolated_singularity_at f z"
    and   has_laurent_expansion_not_essential: "not_essential f z"
  using has_laurent_expansion_isolated_0[OF assms] has_laurent_expansion_not_essential_0[OF assms]
  by (simp_all add: isolated_singularity_at_shift_0 not_essential_shift_0)

lemma has_laurent_expansion_fps:
  assumes "f has_fps_expansion F"
  shows   "f has_laurent_expansion fps_to_fls F"
proof -
  from assms have radius: "0 < fps_conv_radius F" and eval: "∀🪙F z in nhds 0. eval_fps F z = f z"
    by (auto simp: has_fps_expansion_def)
  from eval have eval': "∀🪙F z in at 0. eval_fps F z = f z"
    using eventually_at_filter eventually_mono by fastforce
  moreover have "eventually (λz. z ∈ eball 0 (fps_conv_radius F) - {0}) (at 0)"
    using radius by (intro eventually_at_in_open) (auto simp: zero_ereal_def)
  ultimately have "eventually (λz. eval_fls (fps_to_fls F) z = f z) (at 0)"
    by eventually_elim (auto simp: eval_fps_to_fls)
  thus ?thesis using radius
    by (auto simp: has_laurent_expansion_def)
qed

lemma has_laurent_expansion_const [simp, intro, laurent_expansion_intros]:
  "(λ_. c) has_laurent_expansion fls_const c"
  by (auto simp: has_laurent_expansion_def)

lemma has_laurent_expansion_0 [simp, intro, laurent_expansion_intros]:
  "(λ_. 0) has_laurent_expansion 0"
  by (auto simp: has_laurent_expansion_def)

lemma has_fps_expansion_0_iff: "f has_fps_expansion 0 ⟷ eventually (λz. f z = 0) (nhds 0)"
  by (auto simp: has_fps_expansion_def)

lemma has_laurent_expansion_1 [simp, intro, laurent_expansion_intros]:
  "(λ_. 1) has_laurent_expansion 1"
  by (auto simp: has_laurent_expansion_def)

lemma has_laurent_expansion_numeral [simp, intro, laurent_expansion_intros]:
  "(λ_. numeral n) has_laurent_expansion numeral n"
  by (auto simp: has_laurent_expansion_def)

lemma has_laurent_expansion_fps_X_power [laurent_expansion_intros]:
  "(λx. x ^ n) has_laurent_expansion (fls_X_intpow n)"
  by (auto simp: has_laurent_expansion_def)

lemma has_laurent_expansion_fps_X_power_int [laurent_expansion_intros]:
  "(λx. x powi n) has_laurent_expansion (fls_X_intpow n)"
  by (auto simp: has_laurent_expansion_def)

lemma has_laurent_expansion_fps_X [laurent_expansion_intros]:
  "(λx. x) has_laurent_expansion fls_X"
  by (auto simp: has_laurent_expansion_def)

lemma has_laurent_expansion_cmult_left [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. c * f x) has_laurent_expansion fls_const c * F"
proof -
  from assms have "eventually (λz. z ∈ eball 0 (fls_conv_radius F) - {0}) (at 0)"
    by (intro eventually_at_in_open) (auto simp: has_laurent_expansion_def zero_ereal_def)
  moreover from assms have "eventually (λz. eval_fls F z = f z) (at 0)"
    by (auto simp: has_laurent_expansion_def)
  ultimately have "eventually (λz. eval_fls (fls_const c * F) z = c * f z) (at 0)"
    by eventually_elim (simp_all add: eval_fls_mult)
  with assms show ?thesis
    by (auto simp: has_laurent_expansion_def intro!: less_le_trans[OF _ fls_conv_radius_mult])
qed

lemma has_laurent_expansion_cmult_right [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. f x * c) has_laurent_expansion F * fls_const c"
proof -
  have "F * fls_const c = fls_const c * F"
    by (intro fls_eqI) (auto simp: mult.commute)
  with has_laurent_expansion_cmult_left [OF assms] show ?thesis
    by (simp add: mult.commute)
qed

lemma has_fps_expansion_scaleR [fps_expansion_intros]:
  fixes F :: "'a :: {banach, real_normed_div_algebra, comm_ring_1} fps"
  shows "f has_fps_expansion F ==> (λx. c *🪙R f x) has_fps_expansion fps_const (of_real c) * F"
  unfolding scaleR_conv_of_real by (intro fps_expansion_intros)

lemma has_laurent_expansion_scaleR [laurent_expansion_intros]:
  "f has_laurent_expansion F ==> (λx. c *🪙R f x) has_laurent_expansion fls_const (of_real c) * F"
  unfolding scaleR_conv_of_real by (intro laurent_expansion_intros)

lemma has_laurent_expansion_minus [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. - f x) has_laurent_expansion -F"
proof -
  from assms have "eventually (λx. x ∈ eball 0 (fls_conv_radius F) - {0}) (at 0)"
    by (intro eventually_at_in_open) (auto simp: has_laurent_expansion_def zero_ereal_def)
  moreover from assms have "eventually (λx. eval_fls F x = f x) (at 0)"
    by (auto simp: has_laurent_expansion_def)
  ultimately have "eventually (λx. eval_fls (-F) x = -f x) (at 0)"
    by eventually_elim (auto simp: eval_fls_minus)
  thus ?thesis using assms by (auto simp: has_laurent_expansion_def)
qed

lemma has_laurent_expansion_add [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F" "g has_laurent_expansion G"
  shows   "(λx. f x + g x) has_laurent_expansion F + G"
proof -
  from assms have "0 < min (fls_conv_radius F) (fls_conv_radius G)"
    by (auto simp: has_laurent_expansion_def)
  also have "… ≤ fls_conv_radius (F + G)"
    by (rule fls_conv_radius_add)
  finally have radius: "… > 0" .

  from assms have "eventually (λx. x ∈ eball 0 (fls_conv_radius F) - {0}) (at 0)"
                  "eventually (λx. x ∈ eball 0 (fls_conv_radius G) - {0}) (at 0)"
    by (intro eventually_at_in_open; force simp: has_laurent_expansion_def zero_ereal_def)+
  moreover have "eventually (λx. eval_fls F x = f x) (at 0)"
            and "eventually (λx. eval_fls G x = g x) (at 0)"
    using assms by (auto simp: has_laurent_expansion_def)
  ultimately have "eventually (λx. eval_fls (F + G) x = f x + g x) (at 0)"
    by eventually_elim (auto simp: eval_fls_add)
  with radius show ?thesis by (auto simp: has_laurent_expansion_def)
qed

lemma has_laurent_expansion_diff [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F" "g has_laurent_expansion G"
  shows   "(λx. f x - g x) has_laurent_expansion F - G"
  using has_laurent_expansion_add[of f F "λx. - g x" "-G"] assms
  by (simp add: has_laurent_expansion_minus)

lemma has_laurent_expansion_mult [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F" "g has_laurent_expansion G"
  shows   "(λx. f x * g x) has_laurent_expansion F * G"
proof -
  from assms have "0 < min (fls_conv_radius F) (fls_conv_radius G)"
    by (auto simp: has_laurent_expansion_def)
  also have "… ≤ fls_conv_radius (F * G)"
    by (rule fls_conv_radius_mult)
  finally have radius: "… > 0" .

  from assms have "eventually (λx. x ∈ eball 0 (fls_conv_radius F) - {0}) (at 0)"
                  "eventually (λx. x ∈ eball 0 (fls_conv_radius G) - {0}) (at 0)"
    by (intro eventually_at_in_open; force simp: has_laurent_expansion_def zero_ereal_def)+
  moreover have "eventually (λx. eval_fls F x = f x) (at 0)"
            and "eventually (λx. eval_fls G x = g x) (at 0)"
    using assms by (auto simp: has_laurent_expansion_def)
  ultimately have "eventually (λx. eval_fls (F * G) x = f x * g x) (at 0)"
    by eventually_elim (auto simp: eval_fls_mult)
  with radius show ?thesis by (auto simp: has_laurent_expansion_def)
qed

lemma has_fps_expansion_power [fps_expansion_intros]:
  fixes F :: "'a :: {banach, real_normed_div_algebra, comm_ring_1} fps"
  shows "f has_fps_expansion F ==> (λx. f x ^ m) has_fps_expansion F ^ m"
  by (induction m) (auto intro!: fps_expansion_intros)

lemma has_laurent_expansion_power [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. f x ^ n) has_laurent_expansion F ^ n"
  by (induction n) (auto intro!: laurent_expansion_intros assms)

lemma has_laurent_expansion_sum [laurent_expansion_intros]:
  assumes "∧x. x ∈ I ==> f x has_laurent_expansion F x"
  shows   "(λy. ∑x∈I. f x y) has_laurent_expansion (∑x∈I. F x)"
  using assms by (induction I rule: infinite_finite_induct) (auto intro!: laurent_expansion_intros)

lemma has_laurent_expansion_prod [laurent_expansion_intros]:
  assumes "∧x. x ∈ I ==> f x has_laurent_expansion F x"
  shows   "(λy. ∏x∈I. f x y) has_laurent_expansion (∏x∈I. F x)"
  using assms by (induction I rule: infinite_finite_induct) (auto intro!: laurent_expansion_intros)

lemma has_laurent_expansion_deriv [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "deriv f has_laurent_expansion fls_deriv F"
proof -
  have "eventually (λz. z ∈ eball 0 (fls_conv_radius F) - {0}) (at 0)"
    using assms by (intro eventually_at_in_open)
                   (auto simp: has_laurent_expansion_def zero_ereal_def)
  moreover from assms have "eventually (λz. eval_fls F z = f z) (at 0)"
    by (auto simp: has_laurent_expansion_def)
  then obtain s where "open s" "0 ∈ s" and s: "∧w. w ∈ s - {0} ==> eval_fls F w = f w"
    by (auto simp: eventually_nhds eventually_at_filter)
  hence "eventually (λw. w ∈ s - {0}) (at 0)"
    by (intro eventually_at_in_open) auto
  ultimately have "eventually (λz. eval_fls (fls_deriv F) z = deriv f z) (at 0)"
  proof eventually_elim
    case (elim z)
    hence "eval_fls (fls_deriv F) z = deriv (eval_fls F) z"
      by (simp add: eval_fls_deriv)
    also have "eventually (λw. w ∈ s - {0}) (nhds z)"
      using elim and ‹open s› by (intro eventually_nhds_in_open) auto
    hence "eventually (λw. eval_fls F w = f w) (nhds z)"
      by eventually_elim (use s in auto)
    hence "deriv (eval_fls F) z = deriv f z"
      by (intro deriv_cong_ev refl)
    finally show ?case .
  qed
  with assms show ?thesis
    by (auto simp: has_laurent_expansion_def intro!: less_le_trans[OF _ fls_conv_radius_deriv])
qed

lemma has_laurent_expansion_shift [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. f x * x powi n) has_laurent_expansion (fls_shift (-n) F)"
proof -
  have "eventually (λx. x ∈ eball 0 (fls_conv_radius F) - {0}) (at 0)"
    using assms by (intro eventually_at_in_open) (auto simp: has_laurent_expansion_def zero_ereal_def)
  moreover have "eventually (λx. eval_fls F x = f x) (at 0)"
    using assms by (auto simp: has_laurent_expansion_def)
  ultimately have "eventually (λx. eval_fls (fls_shift (-n) F) x = f x * x powi n) (at 0)"
    by eventually_elim (auto simp: eval_fls_shift assms)
  with assms show ?thesis by (auto simp: has_laurent_expansion_def)
qed

lemma has_laurent_expansion_shift' [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. f x * x powi (-n)) has_laurent_expansion (fls_shift n F)"
  using has_laurent_expansion_shift[OF assms, of "-n"] by simp


lemma has_laurent_expansion_deriv':
  assumes "f has_laurent_expansion F"
  assumes "open A" "0 ∈ A" "∧x. x ∈ A - {0} ==> (f has_field_derivative f' x) (at x)"
  shows   "f' has_laurent_expansion fls_deriv F"
proof -
  have "deriv f has_laurent_expansion fls_deriv F"
    by (intro laurent_expansion_intros assms)
  also have "?this ⟷ ?thesis"
  proof (intro has_laurent_expansion_cong refl)
    have "eventually (λz. z ∈ A - {0}) (at 0)"
      by (intro eventually_at_in_open assms)
    thus "eventually (λz. deriv f z = f' z) (at 0)"
      by eventually_elim (auto intro!: DERIV_imp_deriv assms)
  qed
  finally show ?thesis .
qed

definition laurent_expansion :: "(complex ==> complex) ==> complex ==> complex fls" where
  "laurent_expansion f z =
     (if eventually (λz. f z = 0) (at z) then 0
      else fls_shift (-zorder f z) (fps_to_fls (fps_expansion (zor_poly f z) z)))"

lemma laurent_expansion_cong:
  assumes "eventually (λw. f w = g w) (at z)" "z = z'"
  shows   "laurent_expansion f z = laurent_expansion g z'"
  unfolding laurent_expansion_def
  using zor_poly_cong[OF assms(1,2)] zorder_cong[OF assms] assms
  by (intro if_cong refl) (auto elim: eventually_elim2)

theorem not_essential_has_laurent_expansion_0:
  assumes "isolated_singularity_at f 0" "not_essential f 0"
  shows   "f has_laurent_expansion laurent_expansion f 0"
proof (cases "∃🪙F w in at 0. f w ≠ 0")
  case False
  have "(λ_. 0) has_laurent_expansion 0"
    by simp
  also have "?this ⟷ f has_laurent_expansion 0"
    using False by (intro has_laurent_expansion_cong) (auto simp: frequently_def)
  finally show ?thesis
    using False by (simp add: laurent_expansion_def frequently_def)
next
  case True
  define n where "n = zorder f 0"
  obtain r where r: "zor_poly f 0 0 ≠ 0" "zor_poly f 0 holomorphic_on cball 0 r" "r > 0"
                    "∀w∈cball 0 r - {0}. f w = zor_poly f 0 w * w powi n ∧
                                         zor_poly f 0 w ≠ 0"
    using zorder_exist[OF assms True] unfolding n_def by auto
  have holo: "zor_poly f 0 holomorphic_on ball 0 r"
    by (rule holomorphic_on_subset[OF r(2)]) auto

  define F where "F = fps_expansion (zor_poly f 0) 0"
  have F: "zor_poly f 0 has_fps_expansion F"
    unfolding F_def by (rule has_fps_expansion_fps_expansion[OF _ _ holo]) (use ‹r > 0› in auto)
  have "(λz. zor_poly f 0 z * z powi n) has_laurent_expansion fls_shift (-n) (fps_to_fls F)"
    by (intro laurent_expansion_intros has_laurent_expansion_fps[OF F])
  also have "?this ⟷ f has_laurent_expansion fls_shift (-n) (fps_to_fls F)"
    by (intro has_laurent_expansion_cong refl eventually_mono[OF eventually_at_in_open[of "ball 0 r"]])
       (use r in ‹auto simp: complex_powr_of_int›)
  finally show ?thesis using True
    by (simp add: laurent_expansion_def F_def n_def frequently_def)
qed

lemma not_essential_has_laurent_expansion:
  assumes "isolated_singularity_at f z" "not_essential f z"
  shows   "(λx. f (z + x)) has_laurent_expansion laurent_expansion f z"
proof -
  from assms(1) have iso:"isolated_singularity_at (λx. f (z + x)) 0"
    by (simp add: isolated_singularity_at_shift_0)
  moreover from assms(2) have ness:"not_essential (λx. f (z + x)) 0"
    by (simp add: not_essential_shift_0)
  ultimately have "(λx. f (z + x)) has_laurent_expansion laurent_expansion (λx. f (z + x)) 0"
    by (rule not_essential_has_laurent_expansion_0)

  also have "… = laurent_expansion f z"
  proof (cases "∃🪙F w in at z. f w ≠ 0")
    case False
    then have "∀🪙F w in at z. f w = 0" using not_frequently by force
    then have "laurent_expansion (λx. f (z + x)) 0 = 0"
      by (smt (verit, best) add.commute eventually_at_to_0 eventually_mono
          laurent_expansion_def)
    moreover have "laurent_expansion f z = 0"
      using ‹∀🪙F w in at z. f w = 0› unfolding laurent_expansion_def by auto
    ultimately show ?thesis by auto
  next
    case True
    define df where "df=zor_poly (λx. f (z + x)) 0"
    define g where "g=(λu. u-z)"

    have "fps_expansion df 0
        = fps_expansion (df o g) z"
    proof -
      have "∃🪙F w in at 0. f (z + w) ≠ 0" using True
        by (smt (verit, best) add.commute eventually_at_to_0
            eventually_mono not_frequently)
      from zorder_exist[OF iso ness this,folded df_def]
      obtain r where "r>0" and df_holo:"df holomorphic_on cball 0 r" and "df 0 ≠ 0"
          "∀w∈cball 0 r - {0}.
             f (z + w) = df w * w powi (zorder (λw. f (z + w)) 0) ∧
             df w ≠ 0"
        by auto
      then have df_nz:"∀w∈ball 0 r. df w≠0" by auto

      have "(deriv ^^ n) df 0 = (deriv ^^ n) (df ∘ g) z" for n
        unfolding comp_def g_def
      proof (subst higher_deriv_compose_linear'[where u=1 and c="-z",simplified])
        show "df holomorphic_on ball 0 r"
          using df_holo by auto
        show "open (ball z r)" "open (ball 0 r)" "z ∈ ball z r"
          using ‹r>0› by auto
        show " ∧w. w ∈ ball z r ==> w - z ∈ ball 0 r"
          by (simp add: dist_norm)
      qed auto
      then show ?thesis
        unfolding fps_expansion_def by auto
    qed
    also have "... = fps_expansion (zor_poly f z) z"
    proof (rule fps_expansion_cong)
      have "∀🪙F w in nhds z. zor_poly f z w
                = zor_poly (λu. f (z + u)) 0 (w - z)"
        apply (rule zor_poly_shift)
        using True assms by auto
      then show "∀🪙F w in nhds z. (df ∘ g) w = zor_poly f z w"
        unfolding df_def g_def comp_def
        by (auto elim:eventually_mono)
    qed
    finally show ?thesis unfolding df_def
      by (auto simp: laurent_expansion_def at_to_0[of z]
          eventually_filtermap add_ac zorder_shift')
  qed
  finally show ?thesis .
qed

lemma has_fps_expansion_to_laurent:
  "f has_fps_expansion F ⟷ f has_laurent_expansion fps_to_fls F ∧ f 0 = fps_nth F 0"
proof safe
  assume *: "f has_laurent_expansion fps_to_fls F" "f 0 = fps_nth F 0"
  have "eventually (λz. z ∈ eball 0 (fps_conv_radius F)) (nhds 0)"
    using * by (intro eventually_nhds_in_open) (auto simp: has_laurent_expansion_def zero_ereal_def)
  moreover have "eventually (λz. z ≠ 0 ⟶ eval_fls (fps_to_fls F) z = f z) (nhds 0)"
    using * by (auto simp: has_laurent_expansion_def eventually_at_filter)
  ultimately have "eventually (λz. f z = eval_fps F z) (nhds 0)"
    by eventually_elim
       (auto simp: has_laurent_expansion_def eventually_at_filter eval_fps_at_0 eval_fps_to_fls *(2))
  thus "f has_fps_expansion F"
    using * by (auto simp: has_fps_expansion_def has_laurent_expansion_def eq_commute)
next
  assume "f has_fps_expansion F"
  thus "f 0 = fps_nth F 0"
    by (metis eval_fps_at_0 has_fps_expansion_imp_holomorphic)
qed (auto intro: has_laurent_expansion_fps)

lemma eval_fps_fls_base_factor [simp]:
  assumes "z ≠ 0"
  shows   "eval_fps (fls_base_factor_to_fps F) z = eval_fls F z * z powi -fls_subdegree F"
  using assms unfolding eval_fls_def by (simp add: power_int_minus field_simps)

lemma has_fps_expansion_imp_analytic_0:
  assumes "f has_fps_expansion F"
  shows   "f analytic_on {0}"
  by (meson analytic_at_two assms has_fps_expansion_imp_holomorphic)

lemma has_fps_expansion_imp_analytic:
  assumes "(λx. f (z + x)) has_fps_expansion F"
  shows   "f analytic_on {z}"
proof -
  have "(λx. f (z + x)) analytic_on {0}"
    by (rule has_fps_expansion_imp_analytic_0) fact
  hence "(λx. f (z + x)) ∘ (λx. x - z) analytic_on {z}"
    by (intro analytic_on_compose_gen analytic_intros) auto
  thus ?thesis
    by (simp add: o_def)
qed

lemma is_pole_cong_asymp_equiv:
  assumes "f ∼[at z] g" "z = z'"
  shows   "is_pole f z = is_pole g z'"
  using asymp_equiv_at_infinity_transfer[OF assms(1)]
        asymp_equiv_at_infinity_transfer[OF asymp_equiv_symI[OF assms(1)]] assms(2)
  unfolding is_pole_def by auto

lemma not_is_pole_const [simp]: "¬is_pole (λ_::'a::perfect_space. c :: complex) z"
  using not_tendsto_and_filterlim_at_infinity[of "at z" "λ_::'a. c" c] by (auto simp: is_pole_def)

lemma has_laurent_expansion_imp_is_pole_iff:
  assumes F: "(λx. f (z + x)) has_laurent_expansion F"
  shows   "is_pole f z ⟷ fls_subdegree F < 0"
proof
  assume pole: "is_pole f z"
  have [simp]: "F ≠ 0"
  proof
    assume "F = 0"
    hence "is_pole f z ⟷ is_pole (λ_. 0 :: complex) z" using assms
      by (intro is_pole_cong)
         (auto simp: has_laurent_expansion_def at_to_0[of z] eventually_filtermap add_ac)
    with pole show False
      by simp
  qed

  note pole
  also have "is_pole f z ⟷
             is_pole (λw. fls_nth F (fls_subdegree F) * (w - z) powi fls_subdegree F) z"
    using has_laurent_expansion_imp_asymp_equiv[OF F] by (intro is_pole_cong_asymp_equiv refl)
  also have "… ⟷ is_pole (λw. (w - z) powi fls_subdegree F) z"
    by simp
  finally have pole': … .

  have False if "fls_subdegree F ≥ 0"
  proof -
    have "(λw. (w - z) powi fls_subdegree F) holomorphic_on UNIV"
      using that by (intro holomorphic_intros) auto
    hence "¬is_pole (λw. (w - z) powi fls_subdegree F) z"
      by (meson UNIV_I not_is_pole_holomorphic open_UNIV)
    with pole' show False
      by simp
  qed
  thus "fls_subdegree F < 0"
    by force
qed (use has_laurent_expansion_imp_is_pole[OF assms] in auto)

lemma analytic_at_imp_has_fps_expansion_0:
  assumes "f analytic_on {0}"
  shows   "f has_fps_expansion fps_expansion f 0"
  using assms has_fps_expansion_fps_expansion analytic_at by fast

lemma analytic_at_imp_has_fps_expansion:
  assumes "f analytic_on {z}"
  shows   "(λx. f (z + x)) has_fps_expansion fps_expansion f z"
proof -
  have "f ∘ (λx. z + x) analytic_on {0}"
    by (intro analytic_on_compose_gen[OF _ assms] analytic_intros) auto
  hence "(f ∘ (λx. z + x)) has_fps_expansion fps_expansion (f ∘ (λx. z + x)) 0"
    unfolding o_def by (intro analytic_at_imp_has_fps_expansion_0) auto
  also have "… = fps_expansion f z"
    by (simp add: fps_expansion_def higher_deriv_shift_0')
  finally show ?thesis by (simp add: add_ac)
qed

lemma has_laurent_expansion_zorder_0:
  assumes "f has_laurent_expansion F" "F ≠ 0"
  shows   "zorder f 0 = fls_subdegree F"
proof -
  define G where "G = fls_base_factor_to_fps F"
  from assms obtain A where A: "0 ∈ A" "open A" "∧x. x ∈ A - {0} ==> eval_fls F x = f x"
    unfolding has_laurent_expansion_def eventually_at_filter eventually_nhds
    by blast

  show ?thesis
  proof (rule zorder_eqI)
    show "open (A ∩ eball 0 (fls_conv_radius F))" "0 ∈ A ∩ eball 0 (fls_conv_radius F)"
      using assms A by (auto simp: has_laurent_expansion_def zero_ereal_def)
    show "eval_fps G holomorphic_on A ∩ eball 0 (fls_conv_radius F)"
      by (intro holomorphic_intros) (auto simp: fls_conv_radius_altdef G_def)
    show "eval_fps G 0 ≠ 0" using ‹F ≠ 0›
      by (auto simp: eval_fps_at_0 G_def)
  next
    fix w :: complex assume "w ∈ A ∩ eball 0 (fls_conv_radius F)" "w ≠ 0"
    thus "f w = eval_fps G w * (w - 0) powi (fls_subdegree F)"
      using A unfolding G_def
      by (subst eval_fps_fls_base_factor)
         (auto simp: complex_powr_of_int power_int_minus field_simps)
  qed
qed

lemma has_laurent_expansion_zorder:
  assumes "(λw. f (z + w)) has_laurent_expansion F" "F ≠ 0"
  shows   "zorder f z = fls_subdegree F"
  using has_laurent_expansion_zorder_0[OF assms] by (simp add: zorder_shift' add_ac)

lemma has_fps_expansion_zorder_0:
  assumes "f has_fps_expansion F" "F ≠ 0"
  shows   "zorder f 0 = int (subdegree F)"
  using assms has_laurent_expansion_zorder_0[of f "fps_to_fls F"]
  by (auto simp: has_fps_expansion_to_laurent fls_subdegree_fls_to_fps)

lemma has_fps_expansion_zorder:
  assumes "(λw. f (z + w)) has_fps_expansion F" "F ≠ 0"
  shows   "zorder f z = int (subdegree F)"
  using has_fps_expansion_zorder_0[OF assms]
  by (simp add: zorder_shift' add_ac)

lemma has_fps_expansion_fls_base_factor_to_fps:
  assumes "f has_laurent_expansion F"
  defines "n ≡ fls_subdegree F"
  defines "c ≡ fps_nth (fls_base_factor_to_fps F) 0"
  shows   "(λz. if z = 0 then c else f z * z powi -n) has_fps_expansion fls_base_factor_to_fps F"
proof -
  have "(λz. f z * z powi -n) has_laurent_expansion fls_shift (-(-n)) F"
    by (intro laurent_expansion_intros assms)
  also have "fls_shift (-(-n)) F = fps_to_fls (fls_base_factor_to_fps F)"
    by (simp add: n_def fls_shift_nonneg_subdegree)
  also have "(λz. f z * z powi - n) has_laurent_expansion fps_to_fls (fls_base_factor_to_fps F) ⟷
             (λz. if z = 0 then c else f z * z powi -n) has_laurent_expansion fps_to_fls (fls_base_factor_to_fps F)"
    by (intro has_laurent_expansion_cong) (auto simp: eventually_at_filter)
  also have "… ⟷ (λz. if z = 0 then c else f z * z powi -n) has_fps_expansion fls_base_factor_to_fps F"
    by (subst has_fps_expansion_to_laurent) (auto simp: c_def)
  finally show ?thesis .
qed

lemma zero_has_laurent_expansion_imp_eq_0:
  assumes "(λ_. 0) has_laurent_expansion F"
  shows   "F = 0"
proof -
  have "at (0 :: complex) ≠ bot"
    by auto
  moreover have "(λz. if z = 0 then fls_nth F (fls_subdegree F) else 0) has_fps_expansion
          fls_base_factor_to_fps F" (is "?f has_fps_expansion _")
    using has_fps_expansion_fls_base_factor_to_fps[OF assms] by (simp cong: if_cong)
  hence "isCont ?f 0"
    using has_fps_expansion_imp_continuous by blast
  hence "?f ←-0→ fls_nth F (fls_subdegree F)"
    by (auto simp: isCont_def)
  moreover have "?f ←-0→ 0 ⟷ (λ_::complex. 0 :: complex) ←-0→ 0"
    by (intro filterlim_cong) (auto simp: eventually_at_filter)
  hence "?f ←-0→ 0"
    by simp
  ultimately have "fls_nth F (fls_subdegree F) = 0"
    by (rule tendsto_unique)
  thus ?thesis
    by (meson nth_fls_subdegree_nonzero)
qed

lemma has_laurent_expansion_unique:
  assumes "f has_laurent_expansion F" "f has_laurent_expansion G"
  shows   "F = G"
proof -
  from assms have "(λx. f x - f x) has_laurent_expansion F - G"
    by (intro laurent_expansion_intros)
  hence "(λ_. 0) has_laurent_expansion F - G"
    by simp
  hence "F - G = 0"
    by (rule zero_has_laurent_expansion_imp_eq_0)
  thus ?thesis
    by simp
qed

lemma laurent_expansion_eqI:
  assumes "(λx. f (z + x)) has_laurent_expansion F"
  shows   "laurent_expansion f z = F"
  using assms has_laurent_expansion_isolated has_laurent_expansion_not_essential
        has_laurent_expansion_unique not_essential_has_laurent_expansion by blast

lemma laurent_expansion_0_eqI:
  assumes "f has_laurent_expansion F"
  shows   "laurent_expansion f 0 = F"
  using assms laurent_expansion_eqI[of f 0] by simp

lemma has_laurent_expansion_nonzero_imp_eventually_nonzero:
  assumes "f has_laurent_expansion F" "F ≠ 0"
  shows   "eventually (λx. f x ≠ 0) (at 0)"
proof (rule ccontr)
  assume "¬eventually (λx. f x ≠ 0) (at 0)"
  with assms have "eventually (λx. f x = 0) (at 0)"
    by (intro not_essential_frequently_0_imp_eventually_0 has_laurent_expansion_isolated
              has_laurent_expansion_not_essential)
       (auto simp: frequently_def)
  hence "(f has_laurent_expansion 0) ⟷ ((λ_. 0) has_laurent_expansion 0)"
    by (intro has_laurent_expansion_cong) auto
  hence "f has_laurent_expansion 0"
    by simp
  with assms(1) have "F = 0"
    using has_laurent_expansion_unique by blast
  with ‹F ≠ 0› show False
    by contradiction
qed

lemma has_laurent_expansion_eventually_nonzero_iff':
  assumes "f has_laurent_expansion F"
  shows   "eventually (λx. f x ≠ 0) (at 0) ⟷ F ≠ 0 "
proof
  assume "∀🪙F x in at 0. f x ≠ 0"
  moreover have "¬ (∀🪙F x in at 0. f x ≠ 0)" if "F=0"
  proof -
    have "∀🪙F x in at 0. f x = 0"
      using assms that unfolding has_laurent_expansion_def by simp
    then show ?thesis unfolding not_eventually
      by (auto elim:eventually_frequentlyE)
  qed
  ultimately show "F ≠ 0" by auto
qed (simp add:has_laurent_expansion_nonzero_imp_eventually_nonzero[OF assms])

lemma has_laurent_expansion_eventually_nonzero_iff:
  assumes "(λw. f (z+w)) has_laurent_expansion F"
  shows   "eventually (λx. f x ≠ 0) (at z) ⟷ F ≠ 0"
  apply (subst eventually_at_to_0)
  apply (rule has_laurent_expansion_eventually_nonzero_iff')
  using assms by (simp add:add.commute)

lemma has_laurent_expansion_inverse [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F"
  shows   "(λx. inverse (f x)) has_laurent_expansion inverse F"
proof (cases "F = 0")
  case True
  thus ?thesis using assms
    by (auto simp: has_laurent_expansion_def)
next
  case False
  define G where "G = laurent_expansion (λx. inverse (f x)) 0"
  from False have ev: "eventually (λz. f z ≠ 0) (at 0)"
    by (intro has_laurent_expansion_nonzero_imp_eventually_nonzero[OF assms])

  have *: "(λx. inverse (f x)) has_laurent_expansion G" unfolding G_def
    by (intro not_essential_has_laurent_expansion_0 isolated_singularity_at_inverse not_essential_inverse
              has_laurent_expansion_isolated_0[OF assms] has_laurent_expansion_not_essential_0[OF assms])
  have "(λx. f x * inverse (f x)) has_laurent_expansion F * G"
    by (intro laurent_expansion_intros assms *)
  also have "?this ⟷ (λx. 1) has_laurent_expansion F * G"
    by (intro has_laurent_expansion_cong refl eventually_mono[OF ev]) auto
  finally have "(λ_. 1) has_laurent_expansion F * G" .
  moreover have "(λ_. 1) has_laurent_expansion 1"
    by simp
  ultimately have "F * G = 1"
    using has_laurent_expansion_unique by blast
  hence "G = inverse F"
    using inverse_unique by blast
  with * show ?thesis
    by simp
qed

lemma has_laurent_expansion_power_int [laurent_expansion_intros]:
  "f has_laurent_expansion F ==> (λx. f x powi n) has_laurent_expansion (F powi n)"
  by (auto simp: power_int_def intro!: laurent_expansion_intros)


lemma has_fps_expansion_0_analytic_continuation:
  assumes "f has_fps_expansion 0" "f holomorphic_on A"
  assumes "open A" "connected A" "0 ∈ A" "x ∈ A"
  shows   "f x = 0"
proof -
  have "eventually (λz. z ∈ A ∧ f z = 0) (nhds 0)" using assms
    by (intro eventually_conj eventually_nhds_in_open) (auto simp: has_fps_expansion_def)
  then obtain B where B: "open B" "0 ∈ B" "∀z∈B. z ∈ A ∧ f z = 0"
    unfolding eventually_nhds by blast
  show ?thesis
  proof (rule analytic_continuation_open[where f = f and g = "λ_. 0"])
    show "B ≠ {}"
      using ‹open B› B by auto
    show "connected A"
      using assms by auto
  qed (use assms B in auto)
qed

lemma has_laurent_expansion_0_analytic_continuation:
  assumes "f has_laurent_expansion 0" "f holomorphic_on A - {0}"
  assumes "open A" "connected A" "0 ∈ A" "x ∈ A - {0}"
  shows   "f x = 0"
proof -
  have "eventually (λz. z ∈ A - {0} ∧ f z = 0) (at 0)" using assms
    by (intro eventually_conj eventually_at_in_open) (auto simp: has_laurent_expansion_def)
  then obtain B where B: "open B" "0 ∈ B" "∀z∈B - {0}. z ∈ A - {0} ∧ f z = 0"
    unfolding eventually_at_filter eventually_nhds by blast
  show ?thesis
  proof (rule analytic_continuation_open[where f = f and g = "λ_. 0"])
    show "B - {0} ≠ {}"
      using ‹open B› ‹0 ∈ B› by (metis insert_Diff not_open_singleton)
    show "connected (A - {0})"
      using assms by (intro connected_open_delete) auto
  qed (use assms B in auto)
qed

lemma has_fps_expansion_cong:
  assumes "eventually (λx. f x = g x) (nhds 0)" "F = G"
  shows   "f has_fps_expansion F ⟷ g has_fps_expansion G"
  using assms(2) by (auto simp: has_fps_expansion_def elim!: eventually_elim2[OF assms(1)])

lemma zor_poly_has_fps_expansion:
  assumes "f has_laurent_expansion F" "F ≠ 0"
  shows   "zor_poly f 0 has_fps_expansion fls_base_factor_to_fps F"
proof -
  note [simp] = ‹F ≠ 0›
  have "eventually (λz. f z ≠ 0) (at 0)"
    by (rule has_laurent_expansion_nonzero_imp_eventually_nonzero[OF assms])
  hence freq: "frequently (λz. f z ≠ 0) (at 0)"
    by (rule eventually_frequently[rotated]) auto

  have *: "isolated_singularity_at f 0" "not_essential f 0"
    using has_laurent_expansion_isolated_0[OF assms(1)] has_laurent_expansion_not_essential_0[OF assms(1)]
    by auto

  define G where "G = fls_base_factor_to_fps F"
  define n where "n = zorder f 0"
  have n_altdef: "n = fls_subdegree F"
    using has_laurent_expansion_zorder_0 [OF assms(1)] by (simp add: n_def)
  obtain r where r: "zor_poly f 0 0 ≠ 0" "zor_poly f 0 holomorphic_on cball 0 r" "r > 0"
                    "∀w∈cball 0 r - {0}. f w = zor_poly f 0 w * w powi n ∧
                                         zor_poly f 0 w ≠ 0"
    using zorder_exist[OF * freq] unfolding n_def by auto
  obtain r' where r': "r' > 0" "∀x∈ball 0 r'-{0}. eval_fls F x = f x"
    using assms(1) unfolding has_laurent_expansion_def eventually_at_filter eventually_nhds_metric ball_def
    by (auto simp: dist_commute)
  have holo: "zor_poly f 0 holomorphic_on ball 0 r"
    by (rule holomorphic_on_subset[OF r(2)]) auto

  have "(λz. if z = 0 then fps_nth G 0 else f z * z powi -n) has_fps_expansion G"
    unfolding G_def n_altdef by (intro has_fps_expansion_fls_base_factor_to_fps assms)
  also have "?this ⟷ zor_poly f 0 has_fps_expansion G"
  proof (intro has_fps_expansion_cong)
    have "eventually (λz. z ∈ ball 0 (min r r')) (nhds 0)"
      using ‹r > 0› ‹r' > 0› by (intro eventually_nhds_in_open) auto
    thus "∀🪙F x in nhds 0. (if x = 0 then G $ 0 else f x * x powi - n) = zor_poly f 0 x"
    proof eventually_elim
      case (elim w)
      have w: "w ∈ ball 0 r" "w ∈ ball 0 r'"
        using elim by auto
      show ?case
      proof (cases "w = 0")
        case False
        hence "f w = zor_poly f 0 w * w powi n"
          using r w by auto
        thus ?thesis using False
          by (simp add: powr_minus complex_powr_of_int power_int_minus)
      next
        case [simp]: True
        obtain R where R: "R > 0" "R ≤ r" "R ≤ r'" "R ≤ fls_conv_radius F"
          using ‹r > 0› ‹r' > 0› assms(1) unfolding has_laurent_expansion_def
          by (smt (verit, ccfv_SIG) ereal_dense2 ereal_less(2) less_ereal.simps(1) order.strict_implies_order order_trans)
        have "eval_fps G 0 = zor_poly f 0 0"
        proof (rule analytic_continuation_open[where f = "eval_fps G" and g = "zor_poly f 0"])
          show "connected (ball 0 R :: complex set)"
            by auto
          have "of_real R / 2 ∈ ball 0 R - {0 :: complex}"
            using R by auto
          thus "ball 0 R - {0 :: complex} ≠ {}"
            by blast
          show "eval_fps G holomorphic_on ball 0 R"
            using R less_le_trans[OF _ R(4)] unfolding G_def
            by (intro holomorphic_intros) (auto simp: fls_conv_radius_altdef)
          show "zor_poly f 0 holomorphic_on ball 0 R"
            by (rule holomorphic_on_subset[OF holo]) (use R in auto)
          show "eval_fps G z = zor_poly f 0 z" if "z ∈ ball 0 R - {0}" for z
            using that r r' R n_altdef unfolding G_def
            by (subst eval_fps_fls_base_factor)
               (auto simp: complex_powr_of_int field_simps power_int_minus n_def)
        qed (use R in auto)
        hence "zor_poly f 0 0 = fps_nth G 0"
          by (simp add: eval_fps_at_0)
        thus ?thesis by simp
      qed
    qed
  qed (use r' in auto)
  finally show ?thesis
    by (simp add: G_def)
qed

lemma zorder_geI_0:
  assumes "f analytic_on {0}" "f holomorphic_on A" "open A" "connected A" "0 ∈ A" "z ∈ A" "f z ≠ 0"
  assumes "∧k. k < n ==> (deriv ^^ k) f 0 = 0"
  shows   "zorder f 0 ≥ n"
proof -
  define F where "F = fps_expansion f 0"
  from assms have "f has_fps_expansion F"
    unfolding F_def using analytic_at_imp_has_fps_expansion_0 by blast
  hence laurent: "f has_laurent_expansion fps_to_fls F" and [simp]: "f 0 = fps_nth F 0"
    by (simp_all add: has_fps_expansion_to_laurent)

  have [simp]: "F ≠ 0"
  proof
    assume [simp]: "F = 0"
    hence "f z = 0"
    proof (cases "z = 0")
      case False
      have "f has_laurent_expansion 0"
        using laurent by simp
      thus ?thesis
      proof (rule has_laurent_expansion_0_analytic_continuation)
        show "f holomorphic_on A - {0}"
          using assms(2) by (rule holomorphic_on_subset) auto
      qed (use assms False in auto)
    qed auto
    with ‹f z ≠ 0› show False by contradiction
  qed

  have "zorder f 0 = int (subdegree F)"
    using has_laurent_expansion_zorder_0[OF laurent] by (simp add: fls_subdegree_fls_to_fps)
  also have "subdegree F ≥ n"
    using assms by (intro subdegree_geI ‹F ≠ 0›) (auto simp: F_def fps_expansion_def)
  hence "int (subdegree F) ≥ int n"
    by simp
  finally show ?thesis .
qed

lemma zorder_geI:
  assumes "f analytic_on {x}" "f holomorphic_on A" "open A" "connected A" "x ∈ A" "z ∈ A" "f z ≠ 0"
  assumes "∧k. k < n ==> (deriv ^^ k) f x = 0"
  shows   "zorder f x ≥ n"
proof -
  have "zorder f x = zorder (f ∘ (λu. u + x)) 0"
    by (subst zorder_shift) (auto simp: o_def)
  also have "… ≥ n"
  proof (rule zorder_geI_0)
    show "(f ∘ (λu. u + x)) analytic_on {0}"
      by (intro analytic_on_compose_gen[OF _ assms(1)] analytic_intros) auto
    show "f ∘ (λu. u + x) holomorphic_on ((+) (-x)) ` A"
      by (intro holomorphic_on_compose_gen[OF _ assms(2)] holomorphic_intros) auto
    show "connected ((+) (- x) ` A)"
      by (intro connected_continuous_image continuous_intros assms)
    show "open ((+) (- x) ` A)"
      by (intro open_translation assms)
    show "z - x ∈ (+) (- x) ` A"
      using ‹z ∈ A› by auto
    show "0 ∈ (+) (- x) ` A"
      using ‹x ∈ A› by auto
    show "(f ∘ (λu. u + x)) (z - x) ≠ 0"
      using ‹f z ≠ 0› by auto
  next
    fix k :: nat assume "k < n"
    hence "(deriv ^^ k) f x = 0"
      using assms by simp
    also have "(deriv ^^ k) f x = (deriv ^^ k) (f ∘ (+) x) 0"
      by (subst higher_deriv_shift_0) auto
    finally show "(deriv ^^ k) (f ∘ (λu. u + x)) 0 = 0"
      by (subst add.commute) auto
  qed
  finally show ?thesis .
qed

lemma has_laurent_expansion_divide [laurent_expansion_intros]:
  assumes "f has_laurent_expansion F" and "g has_laurent_expansion G"
  shows   "(λx. f x / g x) has_laurent_expansion (F / G)"
proof -
  have "(λx. f x * inverse (g x)) has_laurent_expansion (F * inverse G)"
    by (intro laurent_expansion_intros assms)
  thus ?thesis
    by (simp add: field_simps)
qed

lemma has_laurent_expansion_residue_0:
  assumes "f has_laurent_expansion F"
  shows   "residue f 0 = fls_residue F"
proof (cases "fls_subdegree F ≥ 0")
  case True
  have "residue f 0 = residue (eval_fls F) 0"
    using assms by (intro residue_cong) (auto simp: has_laurent_expansion_def eq_commute)
  also have "… = 0"
    by (rule residue_holo[OF _ _ holomorphic_on_eval_fls[OF order.refl]])
       (use True assms in ‹auto simp: has_laurent_expansion_def zero_ereal_def›)
  also have "… = fls_residue F"
    using True by simp
  finally show ?thesis .
next
  case False
  hence "F ≠ 0"
    by auto
  have *: "zor_poly f 0 has_fps_expansion fls_base_factor_to_fps F"
    by (intro zor_poly_has_fps_expansion False assms ‹F ≠ 0›)

  have "residue f 0 = (deriv ^^ (nat (-zorder f 0) - 1)) (zor_poly f 0) 0 / fact (nat (- zorder f 0) - 1)"
    by (intro residue_pole_order has_laurent_expansion_isolated_0[OF assms]
              has_laurent_expansion_imp_is_pole_0[OF assms]) (use False in auto)
  also have "… = fls_residue F"
    using has_laurent_expansion_zorder_0[OF assms ‹F ≠ 0›] False
    by (subst fps_nth_fps_expansion [OF *, symmetric]) (auto simp: of_nat_diff)
  finally show ?thesis .
qed

lemma has_laurent_expansion_residue:
  assumes "(λx. f (z + x)) has_laurent_expansion F"
  shows   "residue f z = fls_residue F"
  using has_laurent_expansion_residue_0[OF assms] by (simp add: residue_shift_0')

lemma eval_fls_has_laurent_expansion [laurent_expansion_intros]:
  assumes "fls_conv_radius F > 0"
  shows   "eval_fls F has_laurent_expansion F"
  using assms by (auto simp: has_laurent_expansion_def)

lemma fps_expansion_unique_complex:
  fixes F G :: "complex fps"
  assumes "f has_fps_expansion F" "f has_fps_expansion G"
  shows   "F = G"
  using assms unfolding fps_eq_iff by (auto simp: fps_eq_iff fps_nth_fps_expansion)

lemma fps_expansion_eqI:
  assumes "f has_fps_expansion F"
  shows   "fps_expansion f 0 = F"
  using assms unfolding fps_eq_iff
  by (auto simp: fps_eq_iff fps_nth_fps_expansion fps_expansion_def)

lemma holomorphic_on_imp_fps_conv_radius_ge:
  assumes "f has_fps_expansion F" "f holomorphic_on eball 0 r"
  shows   "fps_conv_radius F ≥ r"
proof -
  define n where "n = subdegree F"
  have "fps_conv_radius (fps_expansion f 0) ≥ r"
    by (intro conv_radius_fps_expansion assms)
  also have "fps_expansion f 0 = F"
    using assms by (intro fps_expansion_eqI)
  finally show ?thesis
    by simp
qed

lemma has_fps_expansion_imp_eval_fps_eq:
  assumes "f has_fps_expansion F" "norm z < r"
  assumes "f holomorphic_on ball 0 r"
  shows   "eval_fps F z = f z"
proof -
  have [simp]: "fps_expansion f 0 = F"
    by (rule fps_expansion_eqI) fact
  have *: "f holomorphic_on eball 0 (ereal r)"
    using assms by simp
  from conv_radius_fps_expansion[OF *] have "fps_conv_radius F ≥ ereal r"
    by simp
  have "eval_fps (fps_expansion f 0) z = f (0 + z)"
    by (rule eval_fps_expansion'[OF *]) (use assms in auto)
  thus ?thesis
    by simp
qed

lemma has_fps_expansion_imp_sums_complex:
  fixes F :: "complex fps"
  assumes "f has_fps_expansion F" "f holomorphic_on eball 0 r" "ereal (norm z) < r"
  shows   "(λn. fps_nth F n * z ^ n) sums f z"
proof -
  have r: "fps_conv_radius F ≥ r"
    using assms(1,2) by (rule holomorphic_on_imp_fps_conv_radius_ge)
  from assms obtain R where R: "norm z < R" "ereal R < r"
    using ereal_dense2 less_ereal.simps(1) by blast
  have z: "norm z < fps_conv_radius F"
    using r R assms(3) by order

  have "summable (λn. fps_nth F n * z ^ n)"
    by (rule summable_fps) (use z in auto)
  moreover have "eval_fps F z = f z"
  proof (rule has_fps_expansion_imp_eval_fps_eq[where r = R])
    have *: "ereal (norm z) < r" if "norm z < R" for z :: complex
      using that R ereal_le_less less_imp_le by blast
    show "f holomorphic_on ball 0 R"
      using assms(2) by (rule holomorphic_on_subset) (use * in auto)
  qed (use R assms(1) in auto)
  ultimately show ?thesis
    unfolding eval_fps_def sums_iff by simp
qed

lemma fls_conv_radius_ge:
  assumes "f has_laurent_expansion F"
  assumes "f holomorphic_on eball 0 r - {0}"
  shows   "fls_conv_radius F ≥ r"
proof -
  define n where "n = fls_subdegree F"
  define G where "G = fls_base_factor_to_fps F"
  define g where "g = (λz. if z = 0 then fps_nth G 0 else f z * z powi -n)"
  have G: "g has_fps_expansion G"
    unfolding G_def g_def n_def
    by (intro has_fps_expansion_fls_base_factor_to_fps assms)
  have "(λz. f z * z powi -n) holomorphic_on eball 0 r - {0}"
    by (intro holomorphic_intros assms) auto
  also have "?this ⟷ g holomorphic_on eball 0 r - {0}"
    by (intro holomorphic_cong) (auto simp: g_def)
  finally have "g analytic_on eball 0 r - {0}"
    by (subst analytic_on_open) auto
  moreover have "g analytic_on {0}"
    using G has_fps_expansion_imp_analytic_0 by auto
  ultimately have "g analytic_on (eball 0 r - {0} ∪ {0})"
    by (subst analytic_on_Un) auto
  hence "g analytic_on eball 0 r"
    by (rule analytic_on_subset) auto
  hence "g holomorphic_on eball 0 r"
    by (subst (asm) analytic_on_open) auto
  hence "fps_conv_radius (fps_expansion g 0) ≥ r"
    by (intro conv_radius_fps_expansion)
  also have "fps_expansion g 0 = G"
    using G by (intro fps_expansion_eqI)
  finally show ?thesis
    by (simp add: fls_conv_radius_altdef G_def)
qed

lemma eval_fls_eqI:
  assumes "f has_laurent_expansion F" "f holomorphic_on eball 0 r - {0}"
  assumes "z ∈ eball 0 r - {0}"
  shows   "eval_fls F z = f z"
proof -
  have conv: "fls_conv_radius F ≥ r"
    by (intro fls_conv_radius_ge[OF assms(1,2)])
  have "(λz. eval_fls F z - f z) has_laurent_expansion F - F"
    using assms by (intro laurent_expansion_intros assms) (auto simp: has_laurent_expansion_def)
  hence "(λz. eval_fls F z - f z) has_laurent_expansion 0"
    by simp
  hence "eval_fls F z - f z = 0"
  proof (rule has_laurent_expansion_0_analytic_continuation)
    have "ereal 0 ≤ ereal (norm z)"
      by simp
    also have "norm z < r"
      using assms by auto
    finally have "r > 0"
      by (simp add: zero_ereal_def)
    thus "open (eball 0 r :: complex set)" "connected (eball 0 r :: complex set)"
         "0 ∈ eball 0 r" "z ∈ eball 0 r - {0}"
      using assms by (auto simp: zero_ereal_def)
  qed (auto intro!: holomorphic_intros assms less_le_trans[OF _ conv] split: if_splits)
  thus ?thesis by simp
qed

lemma fls_nth_as_contour_integral:
  assumes F: "f has_laurent_expansion F"
  assumes holo: "f holomorphic_on ball 0 r - {0}"
  assumes R: "0 < R" "R < r"
  shows "((λz. f z * z powi (-(n+1))) has_contour_integral
            complex_of_real (2 * pi) * i * fls_nth F n) (circlepath 0 R)"
proof -
  define I where "I = (λz. f z * z powi (-(n+1)))"
  have "(I has_contour_integral complex_of_real (2 * pi) * i * residue I 0) (circlepath 0 R)"
  proof (rule base_residue)
    show "open (ball (0::complex) r)" "0 ∈ ball (0::complex) r"
      using R F by (auto simp: has_laurent_expansion_def zero_ereal_def)
  qed (use R in ‹auto intro!: holomorphic_intros holomorphic_on_subset[OF holo]
  simp: I_def split: if_splits›)
  also have "residue I 0 = fls_residue (fls_shift (n + 1) F)"
    unfolding I_def by (intro has_laurent_expansion_residue_0 laurent_expansion_intros F)
  also have "… = fls_nth F n"
    by simp
  finally show ?thesis
    by (simp add: I_def)
qed

lemma tendsto_0_subdegree_iff_0:
  assumes F:"f has_laurent_expansion F" and "F≠0"
  shows "(f ←-0→ 0) ⟷ fls_subdegree F > 0"
proof -
  have ?thesis if "is_pole f 0"
  proof -
    have "fls_subdegree F <0"
      using is_pole_0_imp_neg_fls_subdegree[OF F that] .
    moreover then have "¬ f ←-0→0"
      using ‹is_pole f 0› F at_neq_bot
        has_laurent_expansion_imp_filterlim_infinity_0
        not_tendsto_and_filterlim_at_infinity that
      by blast
    ultimately show ?thesis by auto
  qed
  moreover have ?thesis if "¬is_pole f 0" "∃x. f ←-0→x"
  proof -
    have "fls_subdegree F ≥0"
      using has_laurent_expansion_imp_is_pole_0[OF F] that(1)
      by linarith
    have "f ←-0→0" if "fls_subdegree F > 0"
      using fls_eq0_below_subdegree[OF that]
      by (metis F ‹0 ≤ fls_subdegree F› has_laurent_expansion_imp_tendsto_0)
    moreover have "fls_subdegree F > 0" if "f ←-0→0"
    proof -
      have False if "fls_subdegree F = 0"
      proof -
        have "f ←-0→ fls_nth F 0"
          using has_laurent_expansion_imp_tendsto_0
              [OF F ‹fls_subdegree F ≥0›] .
        then have "fls_nth F 0 = 0" using ‹f ←-0→0›
          using LIM_unique by blast
        then have "F = 0"
          using nth_fls_subdegree_zero_iff ‹fls_subdegree F = 0›
          by metis
        with ‹F≠0› show False by auto
      qed
      with ‹fls_subdegree F ≥0›
      show ?thesis by fastforce
    qed
    ultimately show ?thesis by auto
  qed
  moreover have "is_pole f 0 ∨ (∃x. f ←-0→x)"
  proof -
    have "not_essential f 0"
      using F has_laurent_expansion_not_essential_0 by auto
    then show ?thesis unfolding not_essential_def
      by auto
  qed
  ultimately show ?thesis by auto
qed

lemma tendsto_0_subdegree_iff:
  assumes F: "(λw. f (z+w)) has_laurent_expansion F" and "F ≠ 0"
  shows "(f ←-z→ 0) ⟷ fls_subdegree F > 0"
  apply (subst Lim_at_zero)
  apply (rule tendsto_0_subdegree_iff_0)
  using assms by auto

lemma is_pole_0_deriv_divide_iff:
  assumes F: "f has_laurent_expansion F" and "F ≠ 0"
  shows "is_pole (λx. deriv f x / f x) 0 ⟷ is_pole f 0 ∨ (f ←-0→ 0)"
proof -
  have "(λx. deriv f x / f x) has_laurent_expansion fls_deriv F / F"
    using F by (auto intro:laurent_expansion_intros)

  have "is_pole (λx. deriv f x / f x) 0 ⟷
            fls_subdegree (fls_deriv F / F) < 0"
    apply (rule is_pole_fls_subdegree_iff)
    using F by (auto intro:laurent_expansion_intros)
  also have "... ⟷ is_pole f 0 ∨ (f ←-0→0)"
  proof (cases "fls_subdegree F = 0")
    case True
    then have "fls_subdegree (fls_deriv F / F) ≥ 0"
      by (metis diff_zero div_0 ‹F≠0› fls_deriv_subdegree0
          fls_divide_subdegree)
    moreover then have "¬ is_pole f 0"
      by (metis F True is_pole_0_imp_neg_fls_subdegree less_le)
    moreover have "¬ (f ←-0→0)"
      using tendsto_0_subdegree_iff_0[OF F ‹F≠0›] True by auto
    ultimately show ?thesis by auto
  next
    case False
    then have "fls_deriv F ≠ 0"
      by (metis fls_const_subdegree fls_deriv_eq_0_iff)
    then have "fls_subdegree (fls_deriv F / F) =
              fls_subdegree (fls_deriv F) - fls_subdegree F"
      by (rule fls_divide_subdegree[OF _ ‹F≠0›])
    moreover have "fls_subdegree (fls_deriv F) = fls_subdegree F - 1"
      using fls_subdegree_deriv[OF False] .
    ultimately have "fls_subdegree (fls_deriv F / F) < 0" by auto
    moreover have "f ←-0→ 0 = (0 < fls_subdegree F)"
      using tendsto_0_subdegree_iff_0[OF F ‹F ≠ 0›] .
    moreover have "is_pole f 0 = (fls_subdegree F < 0)"
      using is_pole_fls_subdegree_iff F by auto
    ultimately show ?thesis using False by auto
  qed
  finally show ?thesis .
qed

lemma is_pole_deriv_divide_iff:
  assumes F:"(λw. f (z+w)) has_laurent_expansion F" and "F≠0"
  shows "is_pole (λx. deriv f x / f x) z ⟷ is_pole f z ∨ (f ←-z→0)"
proof -
  define ff df where "ff=(λw. f (z+w))" and "df=(λw. deriv f (z + w))"
  have "is_pole (λx. deriv f x / f x) z
          ⟷ is_pole (λx. deriv ff x / ff x) 0"
    unfolding ff_def df_def
    by (simp add:deriv_shift_0' is_pole_shift_0' comp_def algebra_simps)
  moreover have "is_pole f z ⟷ is_pole ff 0"
    unfolding ff_def by (auto simp:is_pole_shift_0')
  moreover have "(f ←-z→0) ⟷ (ff ←-0→0)"
    unfolding ff_def by (simp add: LIM_offset_zero_iff)
  moreover have "is_pole (λx. deriv ff x / ff x) 0 = (is_pole ff 0 ∨ ff ←-0→ 0)"
    apply (rule is_pole_0_deriv_divide_iff)
    using F ff_def ‹F≠0› by blast+
  ultimately show ?thesis by auto
qed

lemma subdegree_imp_eventually_deriv_nzero_0:
  assumes F:"f has_laurent_expansion F" and "fls_subdegree F≠0"
  shows "eventually (λz. deriv f z ≠ 0) (at 0)"
proof -
  have "deriv f has_laurent_expansion fls_deriv F"
    using has_laurent_expansion_deriv[OF F] .
  moreover have "fls_deriv F≠0"
    using ‹fls_subdegree F≠0›
    by (metis fls_const_subdegree fls_deriv_eq_0_iff)
  ultimately show ?thesis
    using has_laurent_expansion_eventually_nonzero_iff' by blast
qed

lemma subdegree_imp_eventually_deriv_nzero:
  assumes F:"(λw. f (z+w)) has_laurent_expansion F"
      and "fls_subdegree F≠0"
  shows "eventually (λw. deriv f w ≠ 0) (at z)"
proof -
  have "∀🪙F x in at 0. deriv (λw. f (z + w)) x ≠ 0"
    using subdegree_imp_eventually_deriv_nzero_0 assms by auto
  then show ?thesis
    apply (subst eventually_at_to_0)
    by (simp add:deriv_shift_0' comp_def algebra_simps)
qed

lemma has_fps_expansion_imp_asymp_equiv_0:
  fixes f :: "complex ==> complex"
  assumes F: "f has_fps_expansion F"
  defines "n ≡ subdegree F"
  shows   "f ∼[nhds 0] (λz. fps_nth F n * z ^ n)"
proof -
  have F': "f has_laurent_expansion fps_to_fls F"
    using F has_laurent_expansion_fps by blast

  have "f ∼[at 0] (λz. fps_nth F n * z ^ n)"
    using has_laurent_expansion_imp_asymp_equiv_0[OF F']
    by (simp add: fls_subdegree_fls_to_fps n_def)
  moreover have "f 0 = fps_nth F n * 0 ^ n"
    using F by (auto simp: n_def has_fps_expansion_to_laurent power_0_left)
  ultimately show ?thesis
    by (auto simp: asymp_equiv_nhds_iff)
qed

lemma has_fps_expansion_imp_tendsto_0:
  fixes f :: "complex ==> complex"
  assumes "f has_fps_expansion F"
  shows   "(f ---> fps_nth F 0) (nhds 0)"
proof (rule asymp_equiv_tendsto_transfer)
  show "(λz. fps_nth F (subdegree F) * z ^ subdegree F) ∼[nhds 0] f"
    by (rule asymp_equiv_symI, rule has_fps_expansion_imp_asymp_equiv_0) fact
  have "((λz. F $ subdegree F * z ^ subdegree F) ---> F $ 0) (at 0)"
    by (rule tendsto_eq_intros refl | simp)+ (auto simp: power_0_left)
  thus "((λz. F $ subdegree F * z ^ subdegree F) ---> F $ 0) (nhds 0)"
    by (auto simp: tendsto_nhds_iff power_0_left)
qed

lemma has_fps_expansion_imp_0_eq_fps_nth_0:
  assumes "f has_fps_expansion F"
  shows   "f 0 = fps_nth F 0"
proof -
  have "eventually (λx. f x = eval_fps F x) (nhds 0)"
    using assms by (auto simp: has_fps_expansion_def eq_commute)
  then obtain A where "open A" "0 ∈ A" "∀x∈A. f x = eval_fps F x"
    unfolding eventually_nhds by blast
  hence "f 0 = eval_fps F 0"
    by blast
  thus ?thesis
    by (simp add: eval_fps_at_0)
qed

lemma fls_nth_compose_aux:
  assumes "f has_fps_expansion F"
  assumes G: "g has_fps_expansion G" "fps_nth G 0 = 0" "fps_deriv G ≠ 0"
  assumes "(f ∘ g) has_laurent_expansion H"
  shows   "fls_nth H (int n) = fps_nth (fps_compose F G) n"
  using assms(1,5)
proof (induction n arbitrary: f F H rule: less_induct)
  case (less n f F H)
  have [simp]: "g 0 = 0"
    using has_fps_expansion_imp_0_eq_fps_nth_0[OF G(1)] G(2) by simp
  have ana_f: "f analytic_on {0}"
    using less.prems by (meson has_fps_expansion_imp_analytic_0)
  have ana_g: "g analytic_on {0}"
    using G by (meson has_fps_expansion_imp_analytic_0)
  have "(f ∘ g) has_laurent_expansion fps_to_fls (fps_expansion (f ∘ g) 0)"
    by (rule analytic_at_imp_has_fps_expansion_0 analytic_intros has_laurent_expansion_fps
             analytic_on_compose_gen ana_f ana_g)+ auto
  with less.prems have "H = fps_to_fls (fps_expansion (f ∘ g) 0)"
    using has_laurent_expansion_unique by blast
  also have "fls_subdegree … ≥ 0"
    by (simp add: fls_subdegree_fls_to_fps)
  finally have subdeg: "fls_subdegree H ≥ 0" .

  show ?case
  proof (cases "n = 0")
    case [simp]: True
    have lim_g: "g ←-0→ 0"
      using has_laurent_expansion_imp_tendsto_0[of g "fps_to_fls G"] G
      by (auto simp: fls_subdegree_fls_to_fps_gt0 has_fps_expansion_to_laurent)
    have lim_f: "(f ---> fps_nth F 0) (nhds 0)"
      by (intro has_fps_expansion_imp_tendsto_0 less.prems)
    have "(λx. f (g x)) ←-0→ fps_nth F 0"
      by (rule filterlim_compose[OF lim_f lim_g])
    moreover have "(f ∘ g) ←-0→ fls_nth H 0"
      by (intro has_laurent_expansion_imp_tendsto_0 less.prems subdeg)
    ultimately have "fps_nth F 0 = fls_nth H 0"
      using tendsto_unique by (force simp: o_def)
    thus ?thesis
      by simp
  next
    case n: False
    define GH where "GH = (fls_deriv H / fls_deriv (fps_to_fls G))"
    define GH' where "GH' = fls_regpart GH"

    have "(λx. deriv (f ∘ g) x / deriv g x) has_laurent_expansion
          fls_deriv H / fls_deriv (fps_to_fls G)"
      by (intro laurent_expansion_intros less.prems has_laurent_expansion_fps[of _ G] G)
    also have "?this ⟷ (deriv f ∘ g) has_laurent_expansion fls_deriv H / fls_deriv (fps_to_fls G)"
    proof (rule has_laurent_expansion_cong)
      from ana_f obtain r1 where r1: "r1 > 0" "f holomorphic_on ball 0 r1"
        unfolding analytic_on_def by blast
      from ana_g obtain r2 where r2: "r2 > 0" "g holomorphic_on ball 0 r2"
        unfolding analytic_on_def by blast
      have lim_g: "g ←-0→ 0"
        using has_laurent_expansion_imp_tendsto_0[of g "fps_to_fls G"] G
        by (auto simp: fls_subdegree_fls_to_fps_gt0 has_fps_expansion_to_laurent)
      moreover have "open (ball 0 r1)" "0 ∈ ball 0 r1"
        using r1 by auto
      ultimately have "eventually (λx. g x ∈ ball 0 r1) (at 0)"
        unfolding tendsto_def by blast
      moreover have "eventually (λx. deriv g x ≠ 0) (at 0)"
        using G fps_to_fls_eq_0_iff has_fps_expansion_deriv has_fps_expansion_to_laurent
              has_laurent_expansion_nonzero_imp_eventually_nonzero by blast
      moreover have "eventually (λx. x ∈ ball 0 (min r1 r2) - {0}) (at 0)"
        by (intro eventually_at_in_open) (use r1 r2 in auto)
      ultimately show "eventually (λx. deriv (f ∘ g) x / deriv g x = (deriv f ∘ g) x) (at 0)"
      proof eventually_elim
        case (elim x)
        thus ?case using r1 r2
          by (subst deriv_chain)
             (auto simp: field_simps holomorphic_on_def at_within_open[of _ "ball _ _"])
      qed
    qed auto
    finally have GH: "(deriv f ∘ g) has_laurent_expansion GH"
      unfolding GH_def .

    have "(deriv f ∘ g) has_laurent_expansion fps_to_fls (fps_expansion (deriv f ∘ g) 0)"
      by (rule analytic_at_imp_has_fps_expansion_0 analytic_intros has_laurent_expansion_fps
               analytic_on_compose_gen ana_f ana_g)+ auto
    with GH have "GH = fps_to_fls (fps_expansion (deriv f ∘ g) 0)"
      using has_laurent_expansion_unique by blast
    also have "fls_subdegree … ≥ 0"
      by (simp add: fls_subdegree_fls_to_fps)
    finally have subdeg': "fls_subdegree GH ≥ 0" .

    have "deriv f has_fps_expansion fps_deriv F"
      by (intro fps_expansion_intros less.prems)
    from this and GH have IH: "fls_nth GH (int k) = fps_nth (fps_compose (fps_deriv F) G) k"
      if "k < n" for k
      by (intro less.IH that)

    have "fps_nth (fps_compose (fps_deriv F) G) n = (∑i=0..n. of_nat (Suc i) * F $ Suc i * G ^ i $ n)"
      by (simp add: fps_compose_nth)

    have "fps_nth (fps_compose F G) n =
            fps_nth (fps_deriv (fps_compose F G)) (n - 1) / of_nat n"
      using n by (cases n) (auto simp del: of_nat_Suc)
    also have "fps_deriv (fps_compose F G) = fps_compose (fps_deriv F) G * fps_deriv G "
      using G by (subst fps_compose_deriv) auto
    also have "fps_nth … (n - 1) = (∑i=0..n-1. (fps_deriv F oo G) $ i * fps_deriv G $ (n - 1 - i))"
      unfolding fps_mult_nth ..
    also have "… = (∑i=0..n-1. fps_nth GH' i * of_nat (n - i) * G $ (n - i))"
      using n by (intro sum.cong) (auto simp: IH Suc_diff_Suc GH'_def)
    also have "… = (∑i=0..n. fps_nth GH' i * of_nat (n - i) * G $ (n - i))"
      by (intro sum.mono_neutral_left) auto
    also have "… = fps_nth (GH' * Abs_fps (λi. of_nat i * fps_nth G i)) n"
      by (simp add: fps_mult_nth mult_ac)
    also have "Abs_fps (λi. of_nat i * fps_nth G i) = fps_X * fps_deriv G"
      by (simp add: fps_mult_fps_X_deriv_shift)
    also have "fps_nth (GH' * (fps_X * fps_deriv G)) n =
               fls_nth (fps_to_fls (GH' * (fps_X * fps_deriv G))) (int n)"
      by simp
    also have "fps_to_fls (GH' * (fps_X * fps_deriv G)) =
                 GH * fps_to_fls (fps_deriv G) * fls_X"
      using subdeg' by (simp add: mult_ac fls_times_fps_to_fls GH'_def)
    also have "GH * fps_to_fls (fps_deriv G) = fls_deriv H"
      unfolding GH_def using G  by (simp add: fls_deriv_fps_to_fls)
    also have "fls_deriv H * fls_X = fls_shift (-1) (fls_deriv H)"
      using fls_X_times_conv_shift(2) by blast
    finally show ?thesis
      using n by simp
  qed
qed

lemma has_fps_expansion_compose [fps_expansion_intros]:
  fixes f g :: "complex ==> complex"
  assumes F: "f has_fps_expansion F"
  assumes G: "g has_fps_expansion G" "fps_nth G 0 = 0"
  shows   "(f ∘ g) has_fps_expansion fps_compose F G"
proof (cases "fps_deriv G = 0")
  case False
  have [simp]: "g 0 = 0"
    using has_fps_expansion_imp_0_eq_fps_nth_0[OF G(1)] G(2) False by simp
  have ana_f: "f analytic_on {0}"
    using F by (meson has_fps_expansion_imp_analytic_0)
  have ana_g: "g analytic_on {0}"
    using G by (meson has_fps_expansion_imp_analytic_0)
  have fg: "(f ∘ g) has_fps_expansion fps_expansion (f ∘ g) 0"
    by (rule analytic_at_imp_has_fps_expansion_0 analytic_intros
         analytic_on_compose_gen ana_f ana_g)+ auto

  have "fls_nth (fps_to_fls (fps_expansion (f ∘ g) 0)) (int n) = fps_nth (fps_compose F G) n" for n
    by (rule fls_nth_compose_aux has_laurent_expansion_fps F G False fg)+
  hence "fps_expansion (f ∘ g) 0 = fps_compose F G"
    by (simp add: fps_eq_iff)
  thus ?thesis using fg
    by simp
next
  case True
  have [simp]: "f 0 = fps_nth F 0"
    using F by (auto dest: has_fps_expansion_imp_0_eq_fps_nth_0)
  from True have "fps_nth G n = 0" for n
    using G(2) by (cases n) (auto simp del: of_nat_Suc)
  hence [simp]: "G = 0"
    by (auto simp: fps_eq_iff)
  have "(λ_. f 0) has_fps_expansion fps_const (f 0)"
    by (intro fps_expansion_intros)
  also have "eventually (λx. g x = 0) (nhds 0)"
    using G by (auto simp: has_fps_expansion_def)
  hence "(λ_. f 0) has_fps_expansion fps_const (f 0) ⟷ (f ∘ g) has_fps_expansion fps_const (f 0)"
    by (intro has_fps_expansion_cong) (auto elim!: eventually_mono)
  thus ?thesis
    by simp
qed


lemma has_fps_expansion_fps_to_fls:
  assumes "f has_laurent_expansion fps_to_fls F"
  shows   "(λz. if z = 0 then fps_nth F 0 else f z) has_fps_expansion F"
  (is "?f' has_fps_expansion _")
proof -
  have "f has_laurent_expansion fps_to_fls F ⟷ ?f' has_laurent_expansion fps_to_fls F"
    by (intro has_laurent_expansion_cong) (auto simp: eventually_at_filter)
  with assms show ?thesis
    by (auto simp: has_fps_expansion_to_laurent)
qed

lemma has_laurent_expansion_compose [laurent_expansion_intros]:
  fixes f g :: "complex ==> complex"
  assumes F: "f has_laurent_expansion F"
  assumes G: "g has_laurent_expansion fps_to_fls G" "fps_nth G 0 = 0" "G ≠ 0"
  shows   "(f ∘ g) has_laurent_expansion fls_compose_fps F G"
proof -
  from assms have lim_g: "g ←-0→ 0"
    by (subst tendsto_0_subdegree_iff_0[OF G(1)])
       (auto simp: fls_subdegree_fls_to_fps subdegree_pos_iff)
  have ev1: "eventually (λz. g z ≠ 0) (at 0)"
    using ‹G ≠ 0› G(1) fps_to_fls_eq_0_iff has_laurent_expansion_fps
           has_laurent_expansion_nonzero_imp_eventually_nonzero by blast
  moreover have "eventually (λz. z ≠ 0) (at (0 :: complex))"
    by (auto simp: eventually_at_filter)
  ultimately have ev: "eventually (λz. z ≠ 0 ∧ g z ≠ 0) (at 0)"
    by eventually_elim blast
  from ev1 and lim_g have lim_g': "filterlim g (at 0) (at 0)"
    by (auto simp: filterlim_at)
  define g' where "g' = (λz. if z = 0 then fps_nth G 0 else g z)"

  show ?thesis
  proof (cases "F = 0")
    assume [simp]: "F = 0"
    have "eventually (λz. f z = 0) (at 0)"
      using F by (auto simp: has_laurent_expansion_def)
    hence "eventually (λz. f (g z) = 0) (at 0)"
      using lim_g' by (rule eventually_compose_filterlim)
    thus ?thesis
      by (auto simp: has_laurent_expansion_def)
  next
    assume [simp]: "F ≠ 0"
    define n where "n = fls_subdegree F"
    define f' where
      "f' = (λz. if z = 0 then fps_nth (fls_base_factor_to_fps F) 0 else f z * z powi -n)"
    have "((λz. (f' ∘ g') z * g z powi n)) has_laurent_expansion fls_compose_fps F G"
      unfolding f'_def n_def fls_compose_fps_def g'_def
      by (intro fps_expansion_intros laurent_expansion_intros has_fps_expansion_fps_to_fls
                has_fps_expansion_fls_base_factor_to_fps assms has_laurent_expansion_fps)
    also have "?this ⟷ ?thesis"
      by (intro has_laurent_expansion_cong eventually_mono[OF ev])
         (auto simp: f'_def power_int_minus g'_def)
    finally show ?thesis .
  qed
qed

lemma has_laurent_expansion_fls_X_inv [laurent_expansion_intros]:
  "inverse has_laurent_expansion fls_X_inv"
  using has_laurent_expansion_inverse[OF has_laurent_expansion_fps_X]
  by (simp add: fls_inverse_X)

lemma zorder_times_analytic:
  assumes "f analytic_on {z}" "g analytic_on {z}"
  assumes "eventually (λz. f z * g z ≠ 0) (at z)"
  shows   "zorder (λz. f z * g z) z = zorder f z + zorder g z"
proof -
  have *: "(λw. f (z + w)) has_fps_expansion fps_expansion f z"
          "(λw. g (z + w)) has_fps_expansion fps_expansion g z"
          "(λw. f (z + w) * g (z + w)) has_fps_expansion fps_expansion f z * fps_expansion g z"
    by (intro fps_expansion_intros analytic_at_imp_has_fps_expansion assms)+
  have [simp]: "fps_expansion f z ≠ 0"
  proof
    assume "fps_expansion f z = 0"
    hence "eventually (λz. f z * g z = 0) (at z)" using *(1)
      by (auto simp: has_fps_expansion_0_iff nhds_to_0' eventually_filtermap eventually_at_filter
               elim: eventually_mono)
    with assms(3) have "eventually (λz. False) (at z)"
      by eventually_elim auto
    thus False by simp
  qed
  have [simp]: "fps_expansion g z ≠ 0"
  proof
    assume "fps_expansion g z = 0"
    hence "eventually (λz. f z * g z = 0) (at z)" using *(2)
      by (auto simp: has_fps_expansion_0_iff nhds_to_0' eventually_filtermap eventually_at_filter
               elim: eventually_mono)
    with assms(3) have "eventually (λz. False) (at z)"
      by eventually_elim auto
    thus False by simp
  qed
  from *[THEN has_fps_expansion_zorder] show ?thesis
    by auto
qed

lemma zorder_const [simp]: "c ≠ 0 ==> zorder (λ_. c) z = 0"
  by (intro zorder_eqI[where S = UNIV]) auto

lemma zorder_prod_analytic:
  assumes "∧x. x ∈ A ==> f x analytic_on {z}"
  assumes "eventually (λz. (∏x∈A. f x z) ≠ 0) (at z)"
  shows   "zorder (λz. ∏x∈A. f x z) z = (∑x∈A. zorder (f x) z)"
  using assms
proof (induction A rule: infinite_finite_induct)
  case (insert x A)
  have "zorder (λz. f x z * (∏x∈A. f x z)) z = zorder (f x) z + zorder (λz. ∏x∈A. f x z) z"
    using insert.prems insert.hyps by (intro zorder_times_analytic analytic_intros) auto
  also have "zorder (λz. ∏x∈A. f x z) z = (∑x∈A. zorder (f x) z)"
    using insert.prems insert.hyps by (intro insert.IH) (auto elim!: eventually_mono)
  finally show ?case using insert
    by simp
qed auto

lemma zorder_eq_0I:
  assumes "g analytic_on {z}" "g z ≠ 0"
  shows   "zorder g z = 0"
  using analytic_at assms zorder_eqI by fastforce

lemma zorder_pos_iff:
  assumes "f holomorphic_on A" "open A" "z ∈ A" "frequently (λz. f z ≠ 0) (at z)"
  shows   "zorder f z > 0 ⟷ f z = 0"
proof -
  have "f analytic_on {z}"
    using assms analytic_at by blast
  hence *: "(λw. f (z + w)) has_fps_expansion fps_expansion f z"
    using analytic_at_imp_has_fps_expansion by blast
  have nz: "fps_expansion f z ≠ 0"
  proof
    assume "fps_expansion f z = 0"
    hence "eventually (λz. f z = 0) (nhds z)"
      using * by (auto simp: has_fps_expansion_def nhds_to_0' eventually_filtermap add_ac)
    hence "eventually (λz. f z = 0) (at z)"
      by (auto simp: eventually_at_filter elim: eventually_mono)
    with assms show False
      by (auto simp: frequently_def)
  qed
  from has_fps_expansion_zorder[OF * this] have eq: "zorder f z = int (subdegree (fps_expansion f z))"
    by auto
  moreover have "subdegree (fps_expansion f z) = 0 ⟷ fps_nth (fps_expansion f z) 0 ≠ 0"
    using nz by (auto simp: subdegree_eq_0_iff)
  moreover have "fps_nth (fps_expansion f z) 0 = f z"
    by (auto simp: fps_expansion_def)
  ultimately show ?thesis
    by auto
qed

lemma zorder_pos_iff':
  assumes "f analytic_on {z}" "frequently (λz. f z ≠ 0) (at z)"
  shows   "zorder f z > 0 ⟷ f z = 0"
  using analytic_at assms zorder_pos_iff by blast

lemma zorder_ge_0:
  assumes "f analytic_on {z}" "frequently (λz. f z ≠ 0) (at z)"
  shows   "zorder f z ≥ 0"
proof -
  have *: "(λw. f (z + w)) has_laurent_expansion fps_to_fls (fps_expansion f z)"
    using assms by (simp add: analytic_at_imp_has_fps_expansion has_laurent_expansion_fps)
  from * assms(2) have "fps_to_fls (fps_expansion f z) ≠ 0"
    by (auto simp: has_laurent_expansion_def frequently_def at_to_0' eventually_filtermap add_ac)
  with has_laurent_expansion_zorder[OF *] show ?thesis
    by (simp add: fls_subdegree_fls_to_fps)
qed

lemma zorder_eq_0_iff:
  assumes "f analytic_on {z}" "frequently (λw. f w ≠ 0) (at z)"
  shows   "zorder f z = 0 ⟷ f z ≠ 0"
  using assms zorder_eq_0I zorder_pos_iff' by fastforce

lemma zorder_scale:
  assumes "f analytic_on {a * z}" "eventually (λw. f w ≠ 0) (at (a * z))" "a ≠ 0"
  shows "zorder (λw. f (a * w)) z = zorder f (a * z)"
proof -
  from assms(1) obtain r where r: "r > 0" "f holomorphic_on ball (a * z) r"
    by (auto simp: analytic_on_def)
  have *: "open (ball (a * z) r)" "connected (ball (a * z) r)" "a * z ∈ ball (a * z) r"
    using r ‹a ≠ 0› by (auto simp: dist_norm)
  from assms(2) have "eventually (λw. f w ≠ 0 ∧ w ∈ ball (a * z) r - {a * z}) (at (a * z))"
    using ‹r > 0› by (intro eventually_conj eventually_at_in_open) auto
  then obtain z0 where "f z0 ≠ 0 ∧ z0 ∈ ball (a * z) r - {a * z}"
    using eventually_happens[of _ "at (a * z)"] by force
  hence **: "∃w∈ball (a * z) r. f w ≠ 0"
    by blast

  define n where "n = nat (zorder f (a * z))"
  obtain r' where r':
     "(if f (a * z) = 0 then 0 < zorder f (a * z) else zorder f (a * z) = 0)"
     "r' > 0" "cball (a * z) r' ⊆ ball (a * z) r" "zor_poly f (a * z) holomorphic_on cball (a * z) r'"
     "∧w. w ∈ cball (a * z) r' ==>
        f w = zor_poly f (a * z) w * (w - a * z) ^ n ∧ zor_poly f (a * z) w ≠ 0"
    unfolding n_def using zorder_exist_zero[OF r(2) * **] by blast

  show ?thesis
  proof (rule zorder_eqI)
    show "open (ball z (r' / norm a))" "z ∈ ball z (r' / norm a)"
      using r ‹r' > 0› ‹a ≠ 0› by auto
    have "(*) a ` ball z (r' / cmod a) \ cball (a * z) r'"
    proof safe
      fix w assume "w ∈ ball z (r' / cmod a)"
      thus "a * w ∈ cball (a * z) r'"
        using dist_mult_left[of a z w] ‹a ≠ 0› by (auto simp: divide_simps mult_ac)
    qed
    thus "(λw. a ^ n * (zor_poly f (a * z) ∘ (λw. a * w)) w) holomorphic_on ball z (r' / norm a)"
      using ‹a ≠ 0› by (intro holomorphic_on_compose_gen[OF _ r'(4)] holomorphic_intros) auto
    show "a ^ n * (zor_poly f (a * z) ∘ (λw. a * w)) z ≠ 0"
      using r' ‹a ≠ 0› by auto
    show "f (a * w) = a ^ n * (zor_poly f (a * z) ∘ (*) a) w * (w - z) powi (zorder f (a * z))"
      if "w ∈ ball z (r' / norm a)" "w ≠ z" for w
    proof -
      have "f (a * w) = zor_poly f (a * z) (a * w) * (a * (w - z)) ^ n"
        using that r'(5)[of "a * w"] dist_mult_left[of a z w] ‹a ≠ 0› unfolding ring_distribs
        by (auto simp: divide_simps mult_ac)
      also have "… = a ^ n * zor_poly f (a * z) (a * w) * (w - z) ^ n"
        by (subst power_mult_distrib) (auto simp: mult_ac)
      also have "(w - z) ^ n = (w - z) powi of_nat n"
        by simp
      also have "of_nat n = zorder f (a * z)"
        using r'(1) by (auto simp: n_def split: if_splits)
      finally show ?thesis
        unfolding o_def n_def .
    qed
  qed
qed

lemma zorder_compose_aux:
  assumes "isolated_singularity_at f 0" "not_essential f 0"
  assumes G: "g has_fps_expansion G" "G ≠ 0" "g 0 = 0"
  assumes "eventually (λw. f w ≠ 0) (at 0)"
  shows   "zorder (f ∘ g) 0 = zorder f 0 * subdegree G"
proof -
  obtain F where F: "f has_laurent_expansion F"
    using not_essential_has_laurent_expansion_0[OF assms(1,2)] by blast
  have [simp]: "fps_nth G 0 = 0"
   using G ‹g 0 = 0› by (simp add: has_fps_expansion_imp_0_eq_fps_nth_0)
  note [simp] = ‹G ≠ 0› ‹g 0 = 0›
  have [simp]: "F ≠ 0"
    using has_laurent_expansion_eventually_nonzero_iff[of f 0 F] F assms by simp
  have FG: "(f ∘ g) has_laurent_expansion fls_compose_fps F G"
    by (intro has_laurent_expansion_compose has_laurent_expansion_fps F G) auto

  have "zorder (f ∘ g) 0 = fls_subdegree (fls_compose_fps F G)"
    using has_laurent_expansion_zorder_0 [OF FG] by (auto simp: fls_compose_fps_eq_0_iff)
  also have "… = fls_subdegree F * int (subdegree G)"
    by simp
  also have "fls_subdegree F = zorder f 0"
    using has_laurent_expansion_zorder_0 [OF F] by auto
  finally show ?thesis .
qed

lemma zorder_compose:
  assumes "isolated_singularity_at f (g z)" "not_essential f (g z)"
  assumes G: "(λx. g (z + x) - g z) has_fps_expansion G" "G ≠ 0"
  assumes "eventually (λw. f w ≠ 0) (at (g z))"
  shows   "zorder (f ∘ g) z = zorder f (g z) * subdegree G"
proof -
  define f' where "f' = (λw. f (g z + w))"
  define g' where "g' = (λw. g (z + w) - g z)"
  have "zorder f (g z) = zorder f' 0"
    by (simp add: f'_def zorder_shift' add_ac)
  have "zorder (λx. g x - g z) z = zorder g' 0"
    by (simp add: g'_def zorder_shift' add_ac)
  have "zorder (f ∘ g) z = zorder (f' ∘ g') 0"
    by (simp add: zorder_shift' f'_def g'_def add_ac o_def)
  also have "… = zorder f' 0 * int (subdegree G)"
  proof (rule zorder_compose_aux)
    show "isolated_singularity_at f' 0" unfolding f'_def
      using assms has_laurent_expansion_isolated_0 not_essential_has_laurent_expansion by blast
    show "not_essential f' 0" unfolding f'_def
      using assms has_laurent_expansion_not_essential_0 not_essential_has_laurent_expansion by blast
  qed (use assms in ‹auto simp: f'_def g'_def at_to_0' eventually_filtermap add_ac›)
  also have "zorder f' 0 = zorder f (g z)"
    by (simp add: f'_def zorder_shift' add_ac)
  finally show ?thesis .
qed

lemma fps_to_fls_eq_fls_const_iff [simp]: "fps_to_fls F = fls_const c ⟷ F = fps_const c"
  using fps_to_fls_eq_iff by fastforce

lemma zorder_compose':
  assumes "isolated_singularity_at f (g z)" "not_essential f (g z)"
  assumes "g analytic_on {z}"
  assumes "eventually (λw. f w ≠ 0) (at (g z))"
  assumes "eventually (λw. g w ≠ g z) (at z)"
  shows   "zorder (f ∘ g) z = zorder f (g z) * zorder (λx. g x - g z) z"
proof -
  obtain G where G [fps_expansion_intros]: "(λx. g (z + x)) has_fps_expansion G"
    using assms analytic_at_imp_has_fps_expansion by blast
  have G': "(λx. g (z + x) - g z) has_fps_expansion G - fps_const (g z)"
    by (intro fps_expansion_intros)
  hence G'': "(λx. g (z + x) - g z) has_laurent_expansion fps_to_fls (G - fps_const (g z))"
    using has_laurent_expansion_fps by blast
  have nz: "G - fps_const (g z) ≠ 0"
    using has_laurent_expansion_eventually_nonzero_iff[OF G''] assms by auto
  have "zorder (f ∘ g) z = zorder f (g z) * subdegree (G - fps_const (g z))"
  proof (rule zorder_compose)
    show "(λx. g (z + x) - g z) has_fps_expansion G - fps_const (g z)"
      by (intro fps_expansion_intros)
  qed (use assms nz in auto)
  also have "int (subdegree (G - fps_const (g z))) = fls_subdegree (fps_to_fls G - fls_const (g z))"
    by (simp flip: fls_subdegree_fls_to_fps)
  also have "… = zorder (λx. g x - g z) z"
    using has_laurent_expansion_zorder [OF G''] nz by auto
  finally show ?thesis .
qed

lemma analytic_at_cong:
  assumes "eventually (λx. f x = g x) (nhds x)" "x = y"
  shows "f analytic_on {x} ⟷ g analytic_on {y}"
proof -
  have "g analytic_on {x}" if "f analytic_on {x}" "eventually (λx. f x = g x) (nhds x)" for f g
  proof -
    have "(λy. f (x + y)) has_fps_expansion fps_expansion f x"
      by (rule analytic_at_imp_has_fps_expansion) fact
    also have "?this ⟷ (λy. g (x + y)) has_fps_expansion fps_expansion f x"
      using that by (intro has_fps_expansion_cong refl) (auto simp: nhds_to_0' eventually_filtermap)
    finally show ?thesis
      by (rule has_fps_expansion_imp_analytic)
  qed
  from this[of f g] this[of g f] show ?thesis using assms
    by (auto simp: eq_commute)
qed

lemma has_laurent_expansion_sin' [laurent_expansion_intros]:
  "sin has_laurent_expansion fps_to_fls (fps_sin 1)"
  using has_fps_expansion_sin' has_fps_expansion_to_laurent by blast

lemma has_laurent_expansion_cos' [laurent_expansion_intros]:
  "cos has_laurent_expansion fps_to_fls (fps_cos 1)"
  using has_fps_expansion_cos' has_fps_expansion_to_laurent by blast

lemma has_laurent_expansion_sin [laurent_expansion_intros]:
  "(λz. sin (c * z)) has_laurent_expansion fps_to_fls (fps_sin c)"
  by (intro has_laurent_expansion_fps has_fps_expansion_sin)

lemma has_laurent_expansion_cos [laurent_expansion_intros]:
  "(λz. cos (c * z)) has_laurent_expansion fps_to_fls (fps_cos c)"
  by (intro has_laurent_expansion_fps has_fps_expansion_cos)

lemma has_laurent_expansion_tan' [laurent_expansion_intros]:
  "tan has_laurent_expansion fps_to_fls (fps_tan 1)"
  using has_fps_expansion_tan' has_fps_expansion_to_laurent by blast

lemma has_laurent_expansion_tan [laurent_expansion_intros]:
  "(λz. tan (c * z)) has_laurent_expansion fps_to_fls (fps_tan c)"
  by (intro has_laurent_expansion_fps has_fps_expansion_tan)

subsection ‹More Laurent expansions›

lemma has_laurent_expansion_frequently_zero_iff:
  assumes "(λw. f (z + w)) has_laurent_expansion F"
  shows   "frequently (λz. f z = 0) (at z) ⟷ F = 0"
  using assms by (simp add: frequently_def has_laurent_expansion_eventually_nonzero_iff)

lemma has_laurent_expansion_eventually_zero_iff:
  assumes "(λw. f (z + w)) has_laurent_expansion F"
  shows   "eventually (λz. f z = 0) (at z) ⟷ F = 0"
  using assms
  by (metis has_laurent_expansion_frequently_zero_iff has_laurent_expansion_isolated 
            has_laurent_expansion_not_essential laurent_expansion_def 
            not_essential_frequently_0_imp_eventually_0 not_essential_has_laurent_expansion)

lemma has_laurent_expansion_frequently_nonzero_iff:
  assumes "(λw. f (z + w)) has_laurent_expansion F"
  shows   "frequently (λz. f z ≠ 0) (at z) ⟷ F ≠ 0"
  using assms by (metis has_laurent_expansion_eventually_zero_iff not_eventually)

lemma has_laurent_expansion_sum_list [laurent_expansion_intros]:
  assumes "∧x. x ∈ set xs ==> f x has_laurent_expansion F x"
  shows   "(λy. ∑x←xs. f x y) has_laurent_expansion (∑x←xs. F x)"
  using assms by (induction xs) (auto intro!: laurent_expansion_intros)

lemma has_laurent_expansion_prod_list [laurent_expansion_intros]:
  assumes "∧x. x ∈ set xs ==> f x has_laurent_expansion F x"
  shows   "(λy. ∏x←xs. f x y) has_laurent_expansion (∏x←xs. F x)"
  using assms by (induction xs) (auto intro!: laurent_expansion_intros)

lemma has_laurent_expansion_sum_mset [laurent_expansion_intros]:
  assumes "∧x. x ∈# I ==> f x has_laurent_expansion F x"
  shows   "(λy. ∑x∈#I. f x y) has_laurent_expansion (∑x∈#I. F x)"
  using assms by (induction I) (auto intro!: laurent_expansion_intros)

lemma has_laurent_expansion_prod_mset [laurent_expansion_intros]:
  assumes "∧x. x ∈# I ==> f x has_laurent_expansion F x"
  shows   "(λy. ∏x∈#I. f x y) has_laurent_expansion (∏x∈#I. F x)"
  using assms by (induction I) (auto intro!: laurent_expansion_intros)


subsection ‹Formal convergence versus analytic convergence›


text ‹
  The convergence of a sequence of formal power series and the convergence of the functions
  in the complex plane do not imply each other:
 
  🪙 If we have the sequence of constant power series $(1/n)_{n\geq 0}$, this clearly converges
  to the zero function analytically, but as a series of formal power series it is divergent
  (since the 0-th coefficient never stabilises).
 
  🪙 Conversely, the sequence of series $(n! x^n)_{n\geq 0}$ converges formally to $0$,
  but the corresponding sequence of functions diverges for every $x \neq 0$.
 
  However, if the sequence of series converges to some limit series $h$ and the corresponding
  series of functions converges uniformly to some limit function $g(x)$, then $h$ is also a
  series expansion of $g(x)$, i.e.\ in that case, formal and analytic convergence agree.
 ›
proposition uniform_limit_imp_fps_expansion_eq:
  fixes f :: "'a ==> complex fps"
  assumes lim1: "(f ---> h) F"
  assumes lim2: "uniform_limit A (λx z. f' x z) g' F"
  assumes expansions: "eventually (λx. f' x has_fps_expansion f x) F" "g' has_fps_expansion g"
  assumes holo: "eventually (λx. f' x holomorphic_on A) F"
  assumes A: "open A" "0 ∈ A"
  assumes nontriv [simp]: "F ≠ bot"
  shows "g = h"
proof (rule fps_ext)
  fix n :: nat
  have "eventually (λx. fps_nth (f x) n = fps_nth h n) F"
    using lim1 unfolding tendsto_fps_iff by blast
  hence "eventually (λx. (deriv ^^ n) (f' x) 0 / fact n = fps_nth h n) F"
    using expansions(1)
  proof eventually_elim
    case (elim x)
    have "fps_nth (f x) n = (deriv ^^ n) (f' x) 0 / fact n"
      by (rule fps_nth_fps_expansion) (use elim in auto)
    with elim show ?case
      by simp
  qed
  hence "((λx. (deriv ^^ n) (f' x) 0 / fact n) ---> fps_nth h n) F"
    by (simp add: tendsto_eventually)

  moreover have "((λx. (deriv ^^ n) (f' x) 0) ---> (deriv ^^ n) g' 0) F"
    using lim2
  proof (rule higher_deriv_complex_uniform_limit)
    show "eventually (λx. f' x holomorphic_on A) F"
      using holo by eventually_elim auto
  qed (use A in auto)
  hence "((λx. (deriv ^^ n) (f' x) 0 / fact n) ---> (deriv ^^ n) g' 0 / fact n) F"
    by (intro tendsto_divide) auto

  ultimately have "fps_nth h n = (deriv ^^ n) g' 0 / fact n"
    using tendsto_unique[OF nontriv] by blast
  also have "… = fps_nth g n"
    by (rule fps_nth_fps_expansion [symmetric]) fact
  finally show "fps_nth g n = fps_nth h n" ..
qed

end