nevanlinna/Nevanlinna/complexHarmonic.lean

import Mathlib.Analysis.Complex.Basic
import Mathlib.Analysis.Complex.TaylorSeries
import Mathlib.Analysis.Calculus.LineDeriv.Basic
import Mathlib.Analysis.Calculus.ContDiff.Defs
import Mathlib.Analysis.Calculus.FDeriv.Basic
import Mathlib.Analysis.Calculus.FDeriv.Symmetric
import Mathlib.Analysis.SpecialFunctions.Complex.LogDeriv
import Mathlib.Data.Complex.Module
import Mathlib.Data.Complex.Order
import Mathlib.Data.Complex.Exponential
import Mathlib.Data.Fin.Tuple.Basic
import Mathlib.Analysis.RCLike.Basic
import Mathlib.Topology.Algebra.InfiniteSum.Module
import Mathlib.Topology.Instances.RealVectorSpace
import Nevanlinna.cauchyRiemann
import Nevanlinna.laplace
import Nevanlinna.partialDeriv

variable {F : Type*} [NormedAddCommGroup F] [NormedSpace ℝ F]
variable {F₁ : Type*} [NormedAddCommGroup F₁] [NormedSpace ℂ F₁] [CompleteSpace F₁]
variable {G : Type*} [NormedAddCommGroup G] [NormedSpace ℝ G]
variable {G₁ : Type*} [NormedAddCommGroup G₁] [NormedSpace ℂ G₁] [CompleteSpace G₁]


def Harmonic (f : ℂ → F) : Prop :=
  (ContDiff ℝ 2 f) ∧ (∀ z, Complex.laplace f z = 0)


def HarmonicOn (f : ℂ → F) (s : Set ℂ) : Prop :=
  (ContDiffOn ℝ 2 f s) ∧ (∀ z ∈ s, Complex.laplace f z = 0)


theorem HarmonicOn_of_locally_HarmonicOn {f : ℂ → F} {s : Set ℂ} (h : ∀ x ∈ s, ∃ (u : Set ℂ), IsOpen u ∧ x ∈ u ∧ HarmonicOn f (s ∩ u)) :
  HarmonicOn f s := by
  constructor
  · apply contDiffOn_of_locally_contDiffOn
    intro x xHyp
    obtain ⟨u, uHyp⟩  := h x xHyp
    use u
    exact ⟨ uHyp.1, ⟨uHyp.2.1, uHyp.2.2.1⟩⟩
  · intro x xHyp
    obtain ⟨u, uHyp⟩  := h x xHyp
    exact (uHyp.2.2.2) x ⟨xHyp, uHyp.2.1⟩


theorem harmonic_add_harmonic_is_harmonic {f₁ f₂ : ℂ → F} (h₁ : Harmonic f₁) (h₂ : Harmonic f₂) :
  Harmonic (f₁ + f₂) := by
  constructor
  · exact ContDiff.add h₁.1 h₂.1
  · rw [laplace_add h₁.1 h₂.1]
    simp
    intro z
    rw [h₁.2 z, h₂.2 z]
    simp


theorem harmonic_smul_const_is_harmonic {f : ℂ → F} {c : ℝ} (h : Harmonic f) :
  Harmonic (c • f) := by
  constructor
  · exact ContDiff.const_smul c h.1
  · rw [laplace_smul h.1]
    dsimp
    intro z
    rw [h.2 z]
    simp


theorem harmonic_iff_smul_const_is_harmonic {f : ℂ → F} {c : ℝ} (hc : c ≠ 0) :
  Harmonic f ↔ Harmonic (c • f) := by
  constructor
  · exact harmonic_smul_const_is_harmonic
  · nth_rewrite 2 [((eq_inv_smul_iff₀ hc).mpr rfl : f = c⁻¹ • c • f)]
    exact fun a => harmonic_smul_const_is_harmonic a


theorem harmonic_comp_CLM_is_harmonic {f : ℂ → F₁} {l : F₁ →L[ℝ] G} (h : Harmonic f) :
  Harmonic (l ∘ f) := by

  constructor
  · -- Continuous differentiability
    apply ContDiff.comp
    exact ContinuousLinearMap.contDiff l
    exact h.1
  · rw [laplace_compContLin]
    simp
    intro z
    rw [h.2 z]
    simp
    exact ContDiff.restrict_scalars ℝ h.1


theorem harmonicOn_comp_CLM_is_harmonicOn {f : ℂ → F₁} {s : Set ℂ} {l : F₁ →L[ℝ] G} (hs : IsOpen s) (h : HarmonicOn f s) :
  HarmonicOn (l ∘ f) s := by

  constructor
  · -- Continuous differentiability
    apply ContDiffOn.continuousLinearMap_comp
    exact h.1
  · -- Vanishing of Laplace
    intro z zHyp
    rw [laplace_compContLinAt]
    simp
    rw [h.2 z]
    simp
    assumption
    apply ContDiffOn.contDiffAt h.1
    exact IsOpen.mem_nhds hs zHyp


theorem harmonic_iff_comp_CLE_is_harmonic {f : ℂ → F₁} {l : F₁ ≃L[ℝ] G₁} :
  Harmonic f ↔ Harmonic (l ∘ f) := by

  constructor
  · have : l ∘ f = (l : F₁ →L[ℝ] G₁) ∘ f := by rfl
    rw [this]
    exact harmonic_comp_CLM_is_harmonic
  · have : f = (l.symm : G₁ →L[ℝ] F₁) ∘ l ∘ f := by
      funext z
      unfold Function.comp
      simp
    nth_rewrite 2 [this]
    exact harmonic_comp_CLM_is_harmonic


theorem holomorphic_is_harmonic {f : ℂ → F₁} (h : Differentiable ℂ f) :
  Harmonic f := by

  -- f is real C²
  have f_is_real_C2 : ContDiff ℝ 2 f :=
    ContDiff.restrict_scalars ℝ (Differentiable.contDiff h)

  have fI_is_real_differentiable : Differentiable ℝ (partialDeriv ℝ 1 f) := by
    exact (partialDeriv_contDiff ℝ f_is_real_C2 1).differentiable (Submonoid.oneLE.proof_2 ℕ∞)

  constructor
  · -- f is two times real continuously differentiable
    exact f_is_real_C2

  · -- Laplace of f is zero
    unfold Complex.laplace
    rw [CauchyRiemann₄ h]

    -- This lemma says that partial derivatives commute with complex scalar
    -- multiplication. This is a consequence of partialDeriv_compContLin once we
    -- note that complex scalar multiplication is continuous ℝ-linear.
    have : ∀ v, ∀ s : ℂ, ∀ g : ℂ → F₁, Differentiable ℝ g → partialDeriv ℝ v (s • g) = s • (partialDeriv ℝ v g) := by
      intro v s g hg

      -- Present scalar multiplication as a continuous ℝ-linear map. This is
      -- horrible, there must be better ways to do that.
      let sMuls : F₁ →L[ℝ] F₁ :=
      {
        toFun := fun x ↦ s • x
        map_add' := by exact fun x y => DistribSMul.smul_add s x y
        map_smul' := by exact fun m x => (smul_comm ((RingHom.id ℝ) m) s x).symm
        cont := continuous_const_smul s
      }

      -- Bring the goal into a form that is recognized by
      -- partialDeriv_compContLin.
      have : s • g = sMuls ∘ g := by rfl
      rw [this]

      rw [partialDeriv_compContLin ℝ hg]
      rfl

    rw [this]
    rw [partialDeriv_comm f_is_real_C2 Complex.I 1]
    rw [CauchyRiemann₄ h]
    rw [this]
    rw [← smul_assoc]
    simp

    -- Subgoals coming from the application of 'this'
    -- Differentiable ℝ (Real.partialDeriv 1 f)
    exact fI_is_real_differentiable
    -- Differentiable ℝ (Real.partialDeriv 1 f)
    exact fI_is_real_differentiable


theorem re_of_holomorphic_is_harmonic {f : ℂ → ℂ} (h : Differentiable ℂ f) :
  Harmonic (Complex.reCLM ∘ f) := by
  apply harmonic_comp_CLM_is_harmonic
  exact holomorphic_is_harmonic h


theorem im_of_holomorphic_is_harmonic {f : ℂ → ℂ} (h : Differentiable ℂ f) :
  Harmonic (Complex.imCLM ∘ f) := by
  apply harmonic_comp_CLM_is_harmonic
  exact holomorphic_is_harmonic h


theorem antiholomorphic_is_harmonic {f : ℂ → ℂ} (h : Differentiable ℂ f) :
  Harmonic (Complex.conjCLE ∘ f) := by
  apply harmonic_iff_comp_CLE_is_harmonic.1
  exact holomorphic_is_harmonic h


theorem log_normSq_of_holomorphicOn_is_harmonicOn
  {f : ℂ → ℂ}
  {s : Set ℂ}
  (hs : IsOpen s)
  (h₁ : DifferentiableOn ℂ f s)
  (h₂ : ∀ z ∈ s, f z ≠ 0)
  (h₃ : ∀ z ∈ s, f z ∈ Complex.slitPlane) :
  HarmonicOn (Real.log ∘ Complex.normSq ∘ f) s := by

  suffices hyp : HarmonicOn (⇑Complex.ofRealCLM ∘ Real.log ∘ Complex.normSq ∘ f) s from
    (harmonicOn_comp_CLM_is_harmonicOn hs hyp : HarmonicOn (Complex.reCLM ∘ Complex.ofRealCLM ∘ Real.log ∘ Complex.normSq ∘ f) s)

  suffices hyp : HarmonicOn (Complex.log ∘ (((starRingEnd ℂ) ∘ f) * f)) s from by
    have : Complex.ofRealCLM ∘ Real.log ∘ Complex.normSq ∘ f = Complex.log ∘ (((starRingEnd ℂ) ∘ f) * f) := by
      funext z
      simp
      rw [Complex.ofReal_log (Complex.normSq_nonneg (f z))]
      rw [Complex.normSq_eq_conj_mul_self]
    rw [this]
    exact hyp


  -- Suffices to show Harmonic (Complex.log ∘ ⇑(starRingEnd ℂ) ∘ f + Complex.log ∘ f)
  -- THIS IS WHERE WE USE h₃
  have : Complex.log ∘ (⇑(starRingEnd ℂ) ∘ f * f) = Complex.log ∘ ⇑(starRingEnd ℂ) ∘ f + Complex.log ∘ f := by
    unfold Function.comp
    funext z
    simp
    rw [Complex.log_mul_eq_add_log_iff]

    have : Complex.arg ((starRingEnd ℂ) (f z)) = - Complex.arg (f z) := by
      rw [Complex.arg_conj]
      have : ¬ Complex.arg (f z) = Real.pi := by
        exact Complex.slitPlane_arg_ne_pi (h₃ z)
      simp
      tauto
    rw [this]
    simp
    constructor
    · exact Real.pi_pos
    · exact Real.pi_nonneg
    exact (AddEquivClass.map_ne_zero_iff starRingAut).mpr (h₂ z)
    exact h₂ z
  rw [this]

  apply harmonic_add_harmonic_is_harmonic
  have : Complex.log ∘ ⇑(starRingEnd ℂ) ∘ f = Complex.conjCLE ∘ Complex.log ∘ f := by
    funext z
    unfold Function.comp
    rw [Complex.log_conj]
    rfl
    exact Complex.slitPlane_arg_ne_pi (h₃ z)
  rw [this]
  rw [← harmonic_iff_comp_CLE_is_harmonic]

  repeat
    apply holomorphic_is_harmonic
    intro z
    apply DifferentiableAt.comp
    exact Complex.differentiableAt_log (h₃ z)
    exact h₁ z


theorem log_normSq_of_holomorphic_is_harmonic
  {f : ℂ → ℂ}
  (h₁ : Differentiable ℂ f)
  (h₂ : ∀ z, f z ≠ 0)
  (h₃ : ∀ z, f z ∈ Complex.slitPlane) :
  Harmonic (Real.log ∘ Complex.normSq ∘ f) := by

  suffices hyp : Harmonic (⇑Complex.ofRealCLM ∘ Real.log ∘ Complex.normSq ∘ f) from
    (harmonic_comp_CLM_is_harmonic hyp : Harmonic (Complex.reCLM ∘ Complex.ofRealCLM ∘ Real.log ∘ Complex.normSq ∘ f))

  suffices hyp : Harmonic (Complex.log ∘ (((starRingEnd ℂ) ∘ f) * f)) from by
    have : Complex.ofRealCLM ∘ Real.log ∘ Complex.normSq ∘ f = Complex.log ∘ (((starRingEnd ℂ) ∘ f) * f) := by
      funext z
      simp
      rw [Complex.ofReal_log (Complex.normSq_nonneg (f z))]
      rw [Complex.normSq_eq_conj_mul_self]
    rw [this]
    exact hyp


  -- Suffices to show Harmonic (Complex.log ∘ ⇑(starRingEnd ℂ) ∘ f + Complex.log ∘ f)
  -- THIS IS WHERE WE USE h₃
  have : Complex.log ∘ (⇑(starRingEnd ℂ) ∘ f * f) = Complex.log ∘ ⇑(starRingEnd ℂ) ∘ f + Complex.log ∘ f := by
    unfold Function.comp
    funext z
    simp
    rw [Complex.log_mul_eq_add_log_iff]

    have : Complex.arg ((starRingEnd ℂ) (f z)) = - Complex.arg (f z) := by
      rw [Complex.arg_conj]
      have : ¬ Complex.arg (f z) = Real.pi := by
        exact Complex.slitPlane_arg_ne_pi (h₃ z)
      simp
      tauto
    rw [this]
    simp
    constructor
    · exact Real.pi_pos
    · exact Real.pi_nonneg
    exact (AddEquivClass.map_ne_zero_iff starRingAut).mpr (h₂ z)
    exact h₂ z
  rw [this]

  apply harmonic_add_harmonic_is_harmonic
  have : Complex.log ∘ ⇑(starRingEnd ℂ) ∘ f = Complex.conjCLE ∘ Complex.log ∘ f := by
    funext z
    unfold Function.comp
    rw [Complex.log_conj]
    rfl
    exact Complex.slitPlane_arg_ne_pi (h₃ z)
  rw [this]
  rw [← harmonic_iff_comp_CLE_is_harmonic]

  repeat
    apply holomorphic_is_harmonic
    intro z
    apply DifferentiableAt.comp
    exact Complex.differentiableAt_log (h₃ z)
    exact h₁ z


theorem logabs_of_holomorphic_is_harmonic
  {f : ℂ → ℂ}
  (h₁ : Differentiable ℂ f)
  (h₂ : ∀ z, f z ≠ 0)
  (h₃ : ∀ z, f z ∈ Complex.slitPlane) :
  Harmonic (fun z ↦ Real.log ‖f z‖) := by

  -- Suffices: Harmonic (2⁻¹ • Real.log ∘ ⇑Complex.normSq ∘ f)
  have : (fun z ↦ Real.log ‖f z‖) = (2 : ℝ)⁻¹ • (Real.log ∘ Complex.normSq ∘ f) := by
    funext z
    simp
    unfold Complex.abs
    simp
    rw [Real.log_sqrt]
    rw [div_eq_inv_mul (Real.log (Complex.normSq (f z))) 2]
    exact Complex.normSq_nonneg (f z)
  rw [this]

  -- Suffices: Harmonic (Real.log ∘ ⇑Complex.normSq ∘ f)
  apply (harmonic_iff_smul_const_is_harmonic (inv_ne_zero two_ne_zero)).1

  exact log_normSq_of_holomorphic_is_harmonic h₁ h₂ h₃