nevanlinna/Nevanlinna/meromorphicOn.lean

import Mathlib.Analysis.Analytic.Meromorphic
import Nevanlinna.analyticAt
import Nevanlinna.divisor
import Nevanlinna.meromorphicAt
import Nevanlinna.meromorphicOn_divisor
import Nevanlinna.stronglyMeromorphicOn
import Nevanlinna.mathlibAddOn

open scoped Interval Topology
open Real Filter MeasureTheory intervalIntegral

theorem MeromorphicOn.open_of_order_eq_top
  {f : ℂ → ℂ}
  {U : Set ℂ}
  (h₁f : MeromorphicOn f U) :
  IsOpen { u : U | (h₁f u.1 u.2).order = ⊤ } := by

  apply isOpen_iff_forall_mem_open.mpr
  intro z hz
  simp at hz

  have h₁z := hz
  rw [MeromorphicAt.order_eq_top_iff] at hz
  rw [eventually_nhdsWithin_iff] at hz
  rw [eventually_nhds_iff] at hz
  obtain ⟨t', h₁t', h₂t', h₃t'⟩ := hz
  let t : Set U := Subtype.val ⁻¹' t'
  use t
  constructor
  · intro w hw
    simp
    by_cases h₁w : w = z
    · rwa [h₁w]
    · --
      rw [MeromorphicAt.order_eq_top_iff]
      rw [eventually_nhdsWithin_iff]
      rw [eventually_nhds_iff]
      use t' \ {z.1}
      constructor
      · intro y h₁y h₂y
        apply h₁t'
        exact Set.mem_of_mem_diff h₁y
        exact Set.mem_of_mem_inter_right h₁y
      · constructor
        · apply IsOpen.sdiff h₂t'
          exact isClosed_singleton
        · rw [Set.mem_diff]
          constructor
          · exact hw
          · simp
            exact Subtype.coe_ne_coe.mpr h₁w

  · constructor
    · exact isOpen_induced h₂t'
    · exact h₃t'


theorem MeromorphicOn.open_of_order_neq_top
  {f : ℂ → ℂ}
  {U : Set ℂ}
  (h₁f : MeromorphicOn f U) :
  IsOpen { u : U | (h₁f u.1 u.2).order ≠ ⊤ } := by

  apply isOpen_iff_forall_mem_open.mpr
  intro z hz
  simp at hz

  let A := (h₁f z.1 z.2).eventually_eq_zero_or_eventually_ne_zero
  rcases A with h|h
  · rw [← (h₁f z.1 z.2).order_eq_top_iff] at h
    tauto
  · let A := (h₁f z.1 z.2).eventually_analyticAt
    let B := Filter.Eventually.and h A
    rw [eventually_nhdsWithin_iff] at B
    rw [eventually_nhds_iff] at B
    obtain ⟨t', h₁t', h₂t', h₃t'⟩ := B
    let t : Set U := Subtype.val ⁻¹' t'
    use t
    constructor
    · intro w hw
      simp
      by_cases h₁w : w = z
      · rwa [h₁w]
      · let B := h₁t' w hw
        simp at B
        have : (w : ℂ) ≠ (z : ℂ) := by exact Subtype.coe_ne_coe.mpr h₁w
        let C := B this
        let D := C.2.order_eq_zero_iff.2 C.1
        rw [C.2.meromorphicAt_order, D]
        simp
    · constructor
      · exact isOpen_induced h₂t'
      · exact h₃t'


theorem MeromorphicOn.clopen_of_order_eq_top
  {f : ℂ → ℂ}
  {U : Set ℂ}
  (h₁f : MeromorphicOn f U) :
  IsClopen { u : U | (h₁f u.1 u.2).order = ⊤ } := by

  constructor
  · rw [← isOpen_compl_iff]
    exact open_of_order_neq_top h₁f
  · exact open_of_order_eq_top h₁f


theorem MeromorphicOn.order_ne_top
  {U : Set ℂ}
  (hU : IsConnected U)
  (h₁f : StronglyMeromorphicOn f U)
  (h₂f : ∃ u : U, f u ≠ 0) :
  ∀ u : U, (h₁f u u.2).meromorphicAt.order ≠ ⊤ := by

  let A := h₁f.meromorphicOn.clopen_of_order_eq_top
  have : PreconnectedSpace U := by
    apply isPreconnected_iff_preconnectedSpace.mp
    exact IsConnected.isPreconnected hU
  rw [isClopen_iff] at A
  rcases A with h|h
  · intro u
    have : u ∉ (∅ : Set U) := by exact fun a => a
    rw [← h] at this
    simp at this
    tauto
  · obtain ⟨u, hu⟩ := h₂f
    have : u ∈ (Set.univ : Set U) := by trivial
    rw [← h] at this
    simp at this
    rw [(h₁f u u.2).order_eq_zero_iff.2 hu] at this
    tauto