nevanlinna/Nevanlinna/holomorphic.primitive.lean

import Mathlib.Analysis.Complex.TaylorSeries
import Mathlib.Analysis.SpecialFunctions.Integrals
import Mathlib.MeasureTheory.Integral.DivergenceTheorem
import Mathlib.MeasureTheory.Integral.IntervalIntegral
import Mathlib.MeasureTheory.Function.LocallyIntegrable


noncomputable def partialDeriv
  {E : Type*} [NormedAddCommGroup E] [NormedSpace ℝ E]
  {F : Type*} [NormedAddCommGroup F] [NormedSpace ℝ F] : E → (E → F) → (E → F) :=
  fun v ↦ (fun f ↦ (fun w ↦ fderiv ℝ f w v))


theorem partialDeriv_compContLinAt 
  {E : Type*} [NormedAddCommGroup E] [NormedSpace ℂ E]
  {F : Type*} [NormedAddCommGroup F] [NormedSpace ℂ F]
  {G : Type*} [NormedAddCommGroup G] [NormedSpace ℂ G]
  {f : E → F} 
  {l : F →L[ℝ] G} 
  {v : E} 
  {x : E} 
  (h : DifferentiableAt ℝ f x) :
  (partialDeriv v (l ∘ f)) x = (l ∘ partialDeriv v f) x:= by
  unfold partialDeriv
  rw [fderiv.comp x (ContinuousLinearMap.differentiableAt l) h]
  simp

theorem partialDeriv_compCLE
  {E : Type*} [NormedAddCommGroup E] [NormedSpace ℂ E]
  {F : Type*} [NormedAddCommGroup F] [NormedSpace ℂ F]
  {G : Type*} [NormedAddCommGroup G] [NormedSpace ℂ G]
  {f : E → F}
  {l : F ≃L[ℝ] G} {v : E} : partialDeriv v (l ∘ f) = l ∘ partialDeriv v f := by
  funext x
  by_cases hyp : DifferentiableAt ℝ f x
  · let lCLM : F →L[ℝ] G := l
    suffices shyp : partialDeriv v (lCLM ∘ f) x = (lCLM ∘ partialDeriv v f) x from by tauto
    apply partialDeriv_compContLinAt
    exact hyp
  · unfold partialDeriv
    rw [fderiv_zero_of_not_differentiableAt]
    simp
    rw [fderiv_zero_of_not_differentiableAt]
    simp
    exact hyp
    rw [ContinuousLinearEquiv.comp_differentiableAt_iff]
    exact hyp

theorem partialDeriv_smul'₂
  {E : Type*} [NormedAddCommGroup E] [NormedSpace ℂ E]
  {F : Type*} [NormedAddCommGroup F] [NormedSpace ℂ F]
  {f : E → F} {a : ℂ} {v : E} :
  partialDeriv v (a • f) = a • partialDeriv v f := by

  funext w
  by_cases ha : a = 0
  · unfold partialDeriv
    have : a • f = fun y ↦ a • f y := by rfl
    rw [this, ha]
    simp
  · -- Now a is not zero. We present scalar multiplication with a as a continuous linear equivalence.
    let smulCLM : F ≃L[ℝ] F :=
    {
      toFun := fun x ↦ a • x
      map_add' := fun x y => DistribSMul.smul_add a x y
      map_smul' := fun m x => (smul_comm ((RingHom.id ℝ) m) a x).symm
      invFun := fun x ↦ a⁻¹ • x
      left_inv := by
        intro x
        simp
        rw [← smul_assoc, smul_eq_mul, mul_comm, mul_inv_cancel ha, one_smul]
      right_inv := by
        intro x
        simp
        rw [← smul_assoc, smul_eq_mul, mul_inv_cancel ha, one_smul]
      continuous_toFun := continuous_const_smul a
      continuous_invFun := continuous_const_smul a⁻¹
    }

    have : a • f = smulCLM ∘ f := by tauto
    rw [this]
    rw [partialDeriv_compCLE]
    tauto

theorem CauchyRiemann₄
  {F : Type*} [NormedAddCommGroup F] [NormedSpace ℂ F] 
  {f : ℂ → F} : 
  (Differentiable ℂ f) → partialDeriv Complex.I f = Complex.I • partialDeriv 1 f := by
  intro h
  unfold partialDeriv

  conv =>
    left
    intro w
    rw [DifferentiableAt.fderiv_restrictScalars ℝ (h w)]
    simp
    rw [← mul_one Complex.I]
    rw [← smul_eq_mul]
  conv =>
    right
    right
    intro w
    rw [DifferentiableAt.fderiv_restrictScalars ℝ (h w)]
  funext w
  simp

theorem MeasureTheory.integral2_divergence₃
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℝ E] [CompleteSpace E]
  (f g : ℝ × ℝ → E)
  (h₁f : ContDiff ℝ 1 f)
  (h₁g : ContDiff ℝ 1 g)
  (a₁ : ℝ)
  (a₂ : ℝ)
  (b₁ : ℝ)
  (b₂ : ℝ) :
  ∫ (x : ℝ) in a₁..b₁, ∫ (y : ℝ) in a₂..b₂, ((fderiv ℝ f) (x, y)) (1, 0) + ((fderiv ℝ g) (x, y)) (0, 1) = (((∫ (x : ℝ) in a₁..b₁, g (x, b₂)) - ∫ (x : ℝ) in a₁..b₁, g (x, a₂)) + ∫ (y : ℝ) in a₂..b₂, f (b₁, y)) - ∫ (y : ℝ) in a₂..b₂, f (a₁, y) := by

  apply integral2_divergence_prod_of_hasFDerivWithinAt_off_countable f g (fderiv ℝ f) (fderiv ℝ g) a₁ a₂ b₁ b₂ ∅
  exact Set.countable_empty
  -- ContinuousOn f (Set.uIcc a₁ b₁ ×ˢ Set.uIcc a₂ b₂)
  exact h₁f.continuous.continuousOn
  --
  exact h₁g.continuous.continuousOn
  --
  rw [Set.diff_empty]
  intro x _
  exact DifferentiableAt.hasFDerivAt ((h₁f.differentiable le_rfl) x)
  --
  rw [Set.diff_empty]
  intro y _
  exact DifferentiableAt.hasFDerivAt ((h₁g.differentiable le_rfl) y)
  --
  apply ContinuousOn.integrableOn_compact
  apply IsCompact.prod
  exact isCompact_uIcc
  exact isCompact_uIcc
  apply ContinuousOn.add
  apply Continuous.continuousOn
  exact Continuous.clm_apply (ContDiff.continuous_fderiv h₁f le_rfl) continuous_const
  apply Continuous.continuousOn
  exact Continuous.clm_apply (ContDiff.continuous_fderiv h₁g le_rfl) continuous_const


theorem integral_divergence₄
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℝ E] [CompleteSpace E]
  (f g : ℂ  → E)
  (h₁f : ContDiff ℝ 1 f)
  (h₁g : ContDiff ℝ 1 g)
  (a₁ : ℝ)
  (a₂ : ℝ)
  (b₁ : ℝ)
  (b₂ : ℝ) :
  ∫ (x : ℝ) in a₁..b₁, ∫ (y : ℝ) in a₂..b₂, ((fderiv ℝ f) ⟨x, y⟩ ) 1 + ((fderiv ℝ g) ⟨x, y⟩) Complex.I = (((∫ (x : ℝ) in a₁..b₁, g ⟨x, b₂⟩) - ∫ (x : ℝ) in a₁..b₁, g ⟨x, a₂⟩) + ∫ (y : ℝ) in a₂..b₂, f ⟨b₁, y⟩) - ∫ (y : ℝ) in a₂..b₂, f ⟨a₁, y⟩ := by

  let fr : ℝ × ℝ → E := f ∘ Complex.equivRealProdCLM.symm
  let gr : ℝ × ℝ → E := g ∘ Complex.equivRealProdCLM.symm

  have sfr {x y : ℝ} : f { re := x, im := y } = fr (x, y) := by exact rfl
  have sgr {x y : ℝ} : g { re := x, im := y } = gr (x, y) := by exact rfl
  repeat (conv in f { re := _, im := _ } => rw [sfr])
  repeat (conv in g { re := _, im := _ } => rw [sgr])

  have sfr' {x y : ℝ} {z : ℂ} : (fderiv ℝ f { re := x, im := y }) z = fderiv ℝ fr (x, y) (Complex.equivRealProdCLM z) := by
    rw [fderiv.comp]
    rw [Complex.equivRealProdCLM.symm.fderiv]
    tauto
    apply Differentiable.differentiableAt
    exact h₁f.differentiable le_rfl
    exact Complex.equivRealProdCLM.symm.differentiableAt
  conv in ⇑(fderiv ℝ f { re := _, im := _ }) _ => rw [sfr']

  have sgr' {x y : ℝ} {z : ℂ} : (fderiv ℝ g { re := x, im := y }) z = fderiv ℝ gr (x, y) (Complex.equivRealProdCLM z) := by
    rw [fderiv.comp]
    rw [Complex.equivRealProdCLM.symm.fderiv]
    tauto
    apply Differentiable.differentiableAt
    exact h₁g.differentiable le_rfl
    exact Complex.equivRealProdCLM.symm.differentiableAt
  conv in ⇑(fderiv ℝ g { re := _, im := _ }) _ => rw [sgr']

  apply MeasureTheory.integral2_divergence₃ fr gr _ _ a₁ a₂ b₁ b₂
  -- ContDiff ℝ 1 fr
  exact (ContinuousLinearEquiv.contDiff_comp_iff (ContinuousLinearEquiv.symm Complex.equivRealProdCLM)).mpr h₁f
  -- ContDiff ℝ 1 gr
  exact (ContinuousLinearEquiv.contDiff_comp_iff (ContinuousLinearEquiv.symm Complex.equivRealProdCLM)).mpr h₁g


theorem integral_divergence₅
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (F : ℂ  → E)
  (hF : Differentiable ℂ F)
  (lowerLeft upperRight : ℂ) :
  (∫ (x : ℝ) in lowerLeft.re..upperRight.re, F ⟨x, lowerLeft.im⟩)  + Complex.I • ∫ (x : ℝ) in lowerLeft.im..upperRight.im, F ⟨upperRight.re, x⟩ =
  (∫ (x : ℝ) in lowerLeft.re..upperRight.re, F ⟨x, upperRight.im⟩) + Complex.I • ∫ (x : ℝ) in lowerLeft.im..upperRight.im, F ⟨lowerLeft.re,  x⟩ := by

  let h₁f : ContDiff ℝ 1 F := (hF.contDiff : ContDiff ℂ 1 F).restrict_scalars ℝ

  let h₁g : ContDiff ℝ 1 (-Complex.I • F) := by
    have : -Complex.I • F = fun x ↦ -Complex.I • F x := by rfl
    rw [this]
    apply ContDiff.comp
    exact contDiff_const_smul _
    exact h₁f  

  let A := integral_divergence₄ (-Complex.I • F) F h₁g h₁f lowerLeft.re upperRight.im upperRight.re lowerLeft.im
  have {z : ℂ} : fderiv ℝ F z Complex.I = partialDeriv Complex.I F z := by rfl
  conv at A in (fderiv ℝ F _) _ => rw [this]
  have {z : ℂ} : fderiv ℝ (-Complex.I • F) z 1 = partialDeriv 1 (-Complex.I • F) z := by rfl
  conv at A in (fderiv ℝ (-Complex.I • F) _) _ => rw [this]
  conv at A =>
    left
    arg 1
    intro x
    arg 1
    intro y
    rw [CauchyRiemann₄ hF]
    rw [partialDeriv_smul'₂]
    simp
  simp at A

  have {t₁ t₂ t₃ t₄ : E} : 0 = (t₁ - t₂) + t₃ + t₄ → t₁ + t₃ = t₂ - t₄ := by
    intro hyp
    calc
      t₁ + t₃ = t₁ + t₃ - 0 := by rw [sub_zero (t₁ + t₃)]
      _ = t₁ + t₃ - (t₁ - t₂ + t₃ + t₄) := by rw [hyp]
      _ = t₂ - t₄ := by abel
  let B := this A
  repeat
    rw [intervalIntegral.integral_symm lowerLeft.im upperRight.im] at B
  simp at B
  exact B



noncomputable def primitive
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E] :
  ℂ  → (ℂ → E) → (ℂ → E) := by
  intro z₀
  intro f
  exact fun z ↦ (∫ (x : ℝ) in z₀.re..z.re, f ⟨x, z₀.im⟩) + Complex.I • ∫ (x : ℝ) in z₀.im..z.im, f ⟨z.re, x⟩


theorem primitive_zeroAtBasepoint
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (f : ℂ  → E)
  (z₀ : ℂ) :
  (primitive z₀ f) z₀ = 0 := by
  unfold primitive
  simp


theorem primitive_fderivAtBasepointZero
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (f : ℂ  → E)
  (hf : Continuous f) :
  HasDerivAt (primitive 0 f) (f 0) 0 := by
  unfold primitive
  simp
  apply hasDerivAt_iff_isLittleO.2
  simp
  rw [Asymptotics.isLittleO_iff]
  intro c hc

  have {z : ℂ} {e : E} : z • e = (∫ (_ : ℝ) in (0)..(z.re), e) + Complex.I • ∫ (_ : ℝ) in (0)..(z.im), e:= by
    simp
    rw [smul_comm]
    rw [← smul_assoc]
    simp
    have : z.re • e = (z.re : ℂ) • e := by exact rfl
    rw [this, ← add_smul]
    simp
  conv =>
    left
    intro x
    left
    arg 1
    arg 2
    rw [this]
  have {A B C D :E} : (A + B) - (C + D) = (A - C) + (B - D) := by
    abel
  have t₀ {r : ℝ} : IntervalIntegrable (fun x => f { re := x, im := 0 }) MeasureTheory.volume 0 r := by
    apply Continuous.intervalIntegrable
    apply Continuous.comp
    exact hf
    have : (fun x => ({ re := x, im := 0 } : ℂ)) = Complex.ofRealLI := by rfl
    rw [this]
    continuity
  have t₁ {r : ℝ} : IntervalIntegrable (fun _ => f 0) MeasureTheory.volume 0 r := by
    apply Continuous.intervalIntegrable
    apply Continuous.comp
    exact hf
    fun_prop
  have t₂ {a b : ℝ} : IntervalIntegrable (fun x_1 => f { re := a, im := x_1 }) MeasureTheory.volume 0 b := by
    apply Continuous.intervalIntegrable
    apply Continuous.comp hf
    have : (Complex.mk a) = (fun x => Complex.I • Complex.ofRealCLM x + { re := a, im := 0 }) := by
      funext x
      apply Complex.ext
      rw [Complex.add_re]
      simp
      simp
    rw [this]
    apply Continuous.add
    continuity
    fun_prop
    

  have t₃ {a : ℝ} : IntervalIntegrable (fun _ => f 0) MeasureTheory.volume 0 a := by
    apply Continuous.intervalIntegrable
    apply Continuous.comp
    exact hf
    fun_prop
  conv =>
    left
    intro x
    left
    arg 1
    rw [this]
    rw [← smul_sub]
    rw [← intervalIntegral.integral_sub t₀ t₁]
    rw [← intervalIntegral.integral_sub t₂ t₃]
  rw [Filter.eventually_iff_exists_mem]

  let s := f⁻¹' Metric.ball (f 0) (c / (4 : ℝ))
  have h₁s : IsOpen s := IsOpen.preimage hf Metric.isOpen_ball
  have h₂s : 0 ∈ s := by
    apply Set.mem_preimage.mpr
    apply Metric.mem_ball_self
    linarith

  obtain ⟨ε, h₁ε, h₂ε⟩ := Metric.isOpen_iff.1 h₁s 0 h₂s

  have h₃ε : ∀ y ∈ Metric.ball 0 ε, ‖(f y) - (f 0)‖ < (c / (4 : ℝ)) := by
    intro y hy
    apply mem_ball_iff_norm.mp (h₂ε hy)

  use Metric.ball 0 (ε / (4 : ℝ))
  constructor
  · apply Metric.ball_mem_nhds 0
    linarith
  · intro y hy
    have h₁y : |y.re| < ε / 4 := by
      calc |y.re|
      _ ≤ Complex.abs y := by apply Complex.abs_re_le_abs
      _ < ε / 4 := by
        let A := mem_ball_iff_norm.1 hy
        simp at A
        linarith
    have h₂y : |y.im| < ε / 4 := by
      calc |y.im|
      _ ≤ Complex.abs y := by apply Complex.abs_im_le_abs
      _ < ε / 4 := by
        let A := mem_ball_iff_norm.1 hy
        simp at A
        linarith

    have intervalComputation {x' y' : ℝ} (h : x' ∈ Ι 0 y') : |x'| ≤ |y'| := by
      let A := h.1
      let B := h.2
      rcases le_total 0 y' with hy | hy
      · simp [hy] at A
        simp [hy] at B
        rw [abs_of_nonneg hy]
        rw [abs_of_nonneg (le_of_lt A)]
        exact B
      · simp [hy] at A
        simp [hy] at B
        rw [abs_of_nonpos hy]
        rw [abs_of_nonpos]
        linarith [h.1]
        exact B

    have t₁ : ‖(∫ (x : ℝ) in (0)..(y.re), f { re := x, im := 0 } - f 0)‖ ≤ (c / (4 : ℝ)) * |y.re - 0| := by
      apply intervalIntegral.norm_integral_le_of_norm_le_const
      intro x hx

      have h₁x : |x| < ε / 4 := by
        calc |x|
          _ ≤ |y.re| := intervalComputation hx
          _ < ε / 4 := h₁y
      apply le_of_lt
      apply h₃ε { re := x, im := 0 }
      rw [mem_ball_iff_norm]
      simp
      have : { re := x, im := 0 } = (x : ℂ) := by rfl
      rw [this]
      rw [Complex.abs_ofReal]
      linarith

    have t₂ : ‖∫ (x : ℝ) in (0)..(y.im), f { re := y.re, im := x } - f 0‖ ≤ (c / (4 : ℝ)) * |y.im - 0| := by
      apply intervalIntegral.norm_integral_le_of_norm_le_const
      intro x hx

      have h₁x : |x| < ε / 4 := by
        calc |x|
          _ ≤ |y.im| := intervalComputation hx
          _ < ε / 4 := h₂y

      apply le_of_lt
      apply h₃ε { re := y.re, im := x }
      simp

      calc Complex.abs { re := y.re, im := x }
        _ ≤ |y.re| + |x| := by
          apply Complex.abs_le_abs_re_add_abs_im { re := y.re, im := x }
        _ < ε := by
          linarith

    calc ‖(∫ (x : ℝ) in (0)..(y.re), f { re := x, im := 0 } - f 0) + Complex.I • ∫ (x : ℝ) in (0)..(y.im), f { re := y.re, im := x } - f 0‖
      _ ≤ ‖(∫ (x : ℝ) in (0)..(y.re), f { re := x, im := 0 } - f 0)‖ + ‖Complex.I • ∫ (x : ℝ) in (0)..(y.im), f { re := y.re, im := x } - f 0‖ := by
        apply norm_add_le
      _ ≤ ‖(∫ (x : ℝ) in (0)..(y.re), f { re := x, im := 0 } - f 0)‖ + ‖∫ (x : ℝ) in (0)..(y.im), f { re := y.re, im := x } - f 0‖ := by
        simp
        rw [norm_smul]
        simp
      _ ≤ (c / (4 : ℝ)) * |y.re - 0| + (c / (4 : ℝ)) * |y.im - 0| := by
        apply add_le_add
        exact t₁
        exact t₂
      _ ≤ (c / (4 : ℝ)) * (|y.re| + |y.im|) := by
        simp
        rw [mul_add]
      _ ≤ (c / (4 : ℝ)) * (4 * ‖y‖) := by
        have : |y.re| + |y.im| ≤ 4 * ‖y‖ := by
          calc |y.re| + |y.im|
            _ ≤ ‖y‖ + ‖y‖ := by
              apply add_le_add
              apply Complex.abs_re_le_abs
              apply Complex.abs_im_le_abs
            _ ≤ 4 * ‖y‖ := by
              rw [← two_mul]
              apply mul_le_mul
              linarith
              rfl
              exact norm_nonneg y
              linarith

        apply mul_le_mul
        rfl
        exact this
        apply add_nonneg
        apply abs_nonneg
        apply abs_nonneg
        linarith
      _ ≤ c * ‖y‖ := by
        linarith


theorem primitive_translation
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (f : ℂ  → E)
  (z₀ t : ℂ) :
  primitive z₀ (f ∘ fun z ↦ (z - t)) = ((primitive (z₀ - t) f) ∘ fun z ↦ (z - t)) := by
  funext z
  unfold primitive
  simp

  let g : ℝ → E := fun x ↦ f ( {re := x, im := z₀.im - t.im} )
  have {x : ℝ} : f ({ re := x, im := z₀.im } - t) = g (1*x - t.re) := by
    congr 1
    apply Complex.ext <;> simp
  conv =>
    left
    left
    arg 1
    intro x
    rw [this]
  rw [intervalIntegral.integral_comp_mul_sub g one_ne_zero (t.re)]
  simp

  congr 1
  let g : ℝ → E := fun x ↦ f ( {re := z.re - t.re, im := x} )
  have {x : ℝ} : f ({ re := z.re, im := x} - t) = g (1*x - t.im) := by
    congr 1
    apply Complex.ext <;> simp
  conv =>
    left
    arg 1
    intro x
    rw [this]
  rw [intervalIntegral.integral_comp_mul_sub g one_ne_zero (t.im)]
  simp


theorem primitive_fderivAtBasepoint
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  {z₀ : ℂ}
  (f : ℂ  → E)
  (hf : Continuous f) :
  HasDerivAt (primitive z₀ f) (f z₀) z₀ := by

  let g := f ∘ fun z ↦ z + z₀
  have : Continuous g := by continuity
  let A := primitive_fderivAtBasepointZero g this
  simp at A

  let B := primitive_translation g z₀ z₀
  simp at B
  have : (g ∘ fun z ↦ (z - z₀)) = f := by
    funext z
    dsimp [g]
    simp
  rw [this] at B
  rw [B]
  have : f z₀ = (1 : ℂ) • (f z₀) := by
    exact (MulAction.one_smul (f z₀)).symm
  conv =>
    arg 2
    rw [this]

  apply HasDerivAt.scomp
  simp
  have : g 0 = f z₀ := by simp [g]
  rw [← this]
  exact A
  apply HasDerivAt.sub_const
  have : (fun (x : ℂ) ↦ x) = id := by
    funext x
    simp
  rw [this]
  exact hasDerivAt_id z₀


lemma integrability₁
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (f : ℂ  → E)
  (hf : Differentiable ℂ f)
  (a₁ a₂ b : ℝ) :
  IntervalIntegrable (fun x => f { re := x, im := b }) MeasureTheory.volume a₁ a₂ := by
  apply Continuous.intervalIntegrable
  apply Continuous.comp
  exact Differentiable.continuous hf
  have : ((fun x => { re := x, im := b }) : ℝ → ℂ) = (fun x => Complex.ofRealCLM x + { re := 0, im := b }) := by
    funext x
    apply Complex.ext
    rw [Complex.add_re]
    simp
    rw [Complex.add_im]
    simp
  rw [this]
  continuity


lemma integrability₂
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (f : ℂ  → E)
  (hf : Differentiable ℂ f)
  (a₁ a₂ b : ℝ) :
  IntervalIntegrable (fun x => f { re := b, im := x }) MeasureTheory.volume a₁ a₂ := by
  apply Continuous.intervalIntegrable
  apply Continuous.comp
  exact Differentiable.continuous hf
  have : (Complex.mk b) = (fun x => Complex.I • Complex.ofRealCLM x + { re := b, im := 0 }) := by
    funext x
    apply Complex.ext
    rw [Complex.add_re]
    simp
    simp
  rw [this]
  apply Continuous.add
  continuity
  fun_prop


theorem primitive_additivity
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  (f : ℂ  → E)
  (hf : Differentiable ℂ f)
  (z₀ z₁ : ℂ) :
  primitive z₀ f = fun z ↦ (primitive z₁ f) z + (primitive z₀ f z₁) := by
  funext z
  unfold primitive

  have : (∫ (x : ℝ) in z₀.re..z.re, f { re := x, im := z₀.im }) = (∫ (x : ℝ) in z₀.re..z₁.re, f { re := x, im := z₀.im }) + (∫ (x : ℝ) in z₁.re..z.re, f { re := x, im := z₀.im }) := by
    rw [intervalIntegral.integral_add_adjacent_intervals]   
    apply integrability₁ f hf 
    apply integrability₁ f hf 
  rw [this]

  have : (∫ (x : ℝ) in z₀.im..z.im, f { re := z.re, im := x }) = (∫ (x : ℝ) in z₀.im..z₁.im, f { re := z.re, im := x }) + (∫ (x : ℝ) in z₁.im..z.im, f { re := z.re, im := x }) := by
    rw [intervalIntegral.integral_add_adjacent_intervals]
    apply integrability₂ f hf 
    apply integrability₂ f hf 
  rw [this]
  simp

  let A := integral_divergence₅ f hf ⟨z₁.re, z₀.im⟩ ⟨z.re, z₁.im⟩
  simp at A

  have {a b c d : E} : (b + a) + (c + d) = (a + c) + (b + d) := by
    abel
  rw [this]
  rw [A]
  abel


theorem primitive_fderiv
  {E : Type u} [NormedAddCommGroup E] [NormedSpace ℂ E] [CompleteSpace E]
  {z₀ z : ℂ}
  (f : ℂ  → E)
  (hf : Differentiable ℂ f) :
  HasDerivAt (primitive z₀ f) (f z) z := by
  rw [primitive_additivity f hf z₀ z]
  rw [← add_zero (f z)]
  apply HasDerivAt.add
  apply primitive_fderivAtBasepoint
  exact hf.continuous
  apply hasDerivAt_const