import Mathlib
import Definitions.Def_AutomorphicForm_ModularFundamentalDomain

set_option autoImplicit false
set_option maxHeartbeats 800000

noncomputable section

open MeasureTheory UpperHalfPlane ModularForm Filter CongruenceSubgroup
open scoped MatrixGroups ModularForm ComplexConjugate Modular ENNReal Pointwise

namespace CuspForm.PeterssonCoset

@[simp] theorem mapGL_apply (g : SL(2, ℤ)) (i j : Fin 2) :
    (Matrix.SpecialLinearGroup.mapGL ℝ g : GL (Fin 2) ℝ) i j = ((g i j : ℤ) : ℝ) := by
  simp

theorem val_mapGL_eq (g : SL(2, ℤ)) :
    ((Matrix.SpecialLinearGroup.mapGL ℝ g : GL (Fin 2) ℝ) : Matrix (Fin 2) (Fin 2) ℝ)
      = !![((g 0 0 : ℤ) : ℝ), ((g 0 1 : ℤ) : ℝ); ((g 1 0 : ℤ) : ℝ), ((g 1 1 : ℤ) : ℝ)] := by
  ext i j
  rw [mapGL_apply]
  fin_cases i <;> fin_cases j <;> rfl

theorem mapGL_injective : Function.Injective
    (fun s : SL(2, ℤ) => (Matrix.SpecialLinearGroup.mapGL ℝ s : GL (Fin 2) ℝ)) := by
  intro s t h
  apply Subtype.ext
  ext i j
  have := congrArg (fun g : GL (Fin 2) ℝ => (g : Matrix (Fin 2) (Fin 2) ℝ) i j) h
  simpa using this

theorem det_entries (γ : SL(2, ℤ)) : γ 0 0 * γ 1 1 - γ 0 1 * γ 1 0 = 1 := by
  have h := γ.det_coe
  rwa [Matrix.det_fin_two] at h

theorem hact (γ : SL(2, ℤ)) (x : ℍ) :
    γ • x = (Matrix.SpecialLinearGroup.mapGL ℝ γ : GL (Fin 2) ℝ) • x :=
  MulAction.compHom_smul_def _ γ x

theorem mp_smul_SL (γ : SL(2, ℤ)) : MeasurePreserving (fun x : ℍ => γ • x) volume volume := by
  have hemb : MeasurableEmbedding (fun w : ℍ => γ • w) :=
    (Homeomorph.smul γ (α := ℍ)).measurableEmbedding
  exact ⟨hemb.measurable, by
    ext s hs
    rw [Measure.map_apply hemb.measurable hs, Set.preimage_smul,
      FLT.HyperbolicMeasure.volume_smul_sl2z]⟩

theorem mp_smul_GL (g : GL (Fin 2) ℝ) : MeasurePreserving (fun x : ℍ => g • x) volume volume := by
  have hemb : MeasurableEmbedding (fun w : ℍ => g • w) :=
    (Homeomorph.smul g (α := ℍ)).measurableEmbedding
  exact ⟨hemb.measurable, by
    ext s hs
    rw [Measure.map_apply hemb.measurable hs, Set.preimage_smul,
      FLT.HyperbolicMeasure.volume_smul_gl]⟩

theorem measurable_smul_SL (γ : SL(2, ℤ)) : Measurable (fun x : ℍ => γ • x) :=
  (continuous_const_smul γ).measurable

theorem measurable_smul_GL (g : GL (Fin 2) ℝ) : Measurable (fun x : ℍ => g • x) :=
  (continuous_const_smul g).measurable

section weight

variable (Λ : Subgroup SL(2, ℤ))

def splitEquiv : SL(2, ℤ) ≃ (SL(2, ℤ) ⧸ Λ) × Λ where
  toFun σ := (QuotientGroup.mk σ, ⟨(QuotientGroup.mk σ : SL(2, ℤ) ⧸ Λ).out⁻¹ * σ, by
    rw [← QuotientGroup.eq]; exact QuotientGroup.out_eq' _⟩)
  invFun x := x.1.out * x.2
  left_inv σ := by simp
  right_inv := by
    rintro ⟨q, h⟩
    have hq : (QuotientGroup.mk (q.out * (h : SL(2, ℤ))) : SL(2, ℤ) ⧸ Λ) = q := by
      rw [QuotientGroup.mk_mul_of_mem _ h.2, QuotientGroup.out_eq']
    refine Prod.ext hq (Subtype.ext ?_)
    simp only [hq]
    rw [inv_mul_cancel_left]

theorem tsum_SL_eq (F : SL(2, ℤ) → ℝ≥0∞) :
    ∑' σ : SL(2, ℤ), F σ = ∑' q : SL(2, ℤ) ⧸ Λ, ∑' h : Λ, F (q.out * (h : SL(2, ℤ))) := by
  rw [← (splitEquiv Λ).symm.tsum_eq, ENNReal.tsum_prod']
  rfl

theorem tsum_SL_inv (F : SL(2, ℤ) → ℝ≥0∞) : ∑' σ : SL(2, ℤ), F σ = ∑' σ : SL(2, ℤ), F σ⁻¹ :=
  ((Equiv.inv SL(2, ℤ)).tsum_eq F).symm

theorem tsum_Λ_inv (F : Λ → ℝ≥0∞) : ∑' h : Λ, F h = ∑' h : Λ, F h⁻¹ :=
  ((Equiv.inv Λ).tsum_eq F).symm

variable [Fintype (SL(2, ℤ) ⧸ Λ)]

def wt (x : ℍ) : ℝ≥0∞ :=
  ∑ q : SL(2, ℤ) ⧸ Λ, (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) ((q.out : SL(2, ℤ)) • x)

theorem measurable_wt : Measurable (wt Λ) := by
  refine Finset.measurable_sum _ fun q _ => ?_
  exact (measurable_const.indicator ModularGroup.isClosed_fd.measurableSet).comp
    (measurable_smul_SL _)

theorem indicator_smul (σ : SL(2, ℤ)) (x : ℍ) :
    (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) (σ • x)
      = (σ⁻¹ • (𝒟 : Set ℍ)).indicator (fun _ => (1 : ℝ≥0∞)) x := by
  by_cases h : σ • x ∈ (𝒟 : Set ℍ)
  · rw [Set.indicator_of_mem h, Set.indicator_of_mem (Set.mem_inv_smul_set_iff.mpr h)]
  · rw [Set.indicator_of_notMem h, Set.indicator_of_notMem (fun h' => h (Set.mem_inv_smul_set_iff.mp h'))]

theorem tsum_wt_smul {x : ℍ} (hx : x ∈ FLT.ModularFundamentalDomain.goodSet) :
    ∑' h : Λ, wt Λ ((h : SL(2, ℤ)) • x) = 2 := by
  calc ∑' h : Λ, wt Λ ((h : SL(2, ℤ)) • x)
      = ∑' h : Λ, ∑' q : SL(2, ℤ) ⧸ Λ,
          (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) ((q.out : SL(2, ℤ)) • (h : SL(2, ℤ)) • x) :=
        tsum_congr fun h => by rw [wt, tsum_fintype]
    _ = ∑' q : SL(2, ℤ) ⧸ Λ, ∑' h : Λ,
          (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) ((q.out : SL(2, ℤ)) • (h : SL(2, ℤ)) • x) :=
        ENNReal.tsum_comm
    _ = ∑' q : SL(2, ℤ) ⧸ Λ, ∑' h : Λ,
          (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) (((q.out : SL(2, ℤ)) * h) • x) :=
        tsum_congr fun q => tsum_congr fun h => by rw [mul_smul]
    _ = ∑' σ : SL(2, ℤ), (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) (σ • x) :=
        (tsum_SL_eq Λ (fun σ => (𝒟 : Set ℍ).indicator (fun _ => (1 : ℝ≥0∞)) (σ • x))).symm
    _ = ∑' σ : SL(2, ℤ), (σ⁻¹ • (𝒟 : Set ℍ)).indicator (fun _ => (1 : ℝ≥0∞)) x :=
        tsum_congr fun σ => indicator_smul σ x
    _ = ∑' σ : SL(2, ℤ), (σ • (𝒟 : Set ℍ)).indicator (fun _ => (1 : ℝ≥0∞)) x := by
        rw [tsum_SL_inv (fun σ => (σ • (𝒟 : Set ℍ)).indicator (fun _ => (1 : ℝ≥0∞)) x)]
    _ = 2 := FLT.ModularFundamentalDomain.tsum_indicator_smul_fd hx

theorem ae_tsum_wt_smul : ∀ᵐ x : ℍ, ∑' h : Λ, wt Λ ((h : SL(2, ℤ)) • x) = 2 := by
  filter_upwards [measure_eq_zero_iff_ae_notMem.mp FLT.ModularFundamentalDomain.volume_compl_goodSet]
    with x hx
  exact tsum_wt_smul Λ (by simpa using hx)

theorem tsum_G_eq (U w : ℍ → ℝ≥0∞) (hUinv : ∀ h ∈ Λ, ∀ x : ℍ, U (h • x) = U x) (x : ℍ)
    (hwt : ∀ q : SL(2, ℤ) ⧸ Λ, ∑' h : Λ, w ((h : SL(2, ℤ)) • (q.out⁻¹ : SL(2, ℤ)) • x) = 2) :
    ∑' σ : SL(2, ℤ), U (σ • x) * w (σ • x)
      = 2 * ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) := by
  calc ∑' σ : SL(2, ℤ), U (σ • x) * w (σ • x)
      = ∑' σ : SL(2, ℤ), U (σ⁻¹ • x) * w (σ⁻¹ • x) :=
        tsum_SL_inv (fun σ => U (σ • x) * w (σ • x))
    _ = ∑' q : SL(2, ℤ) ⧸ Λ, ∑' h : Λ,
          U (((q.out : SL(2, ℤ)) * h)⁻¹ • x) * w (((q.out : SL(2, ℤ)) * h)⁻¹ • x) :=
        tsum_SL_eq Λ (fun σ => U (σ⁻¹ • x) * w (σ⁻¹ • x))
    _ = ∑' q : SL(2, ℤ) ⧸ Λ, ∑' h : Λ,
          U ((q.out⁻¹ : SL(2, ℤ)) • x) * w (((h⁻¹ : Λ) : SL(2, ℤ)) • (q.out⁻¹ : SL(2, ℤ)) • x) := by
        refine tsum_congr fun q => tsum_congr fun h => ?_
        have h1 : ((q.out : SL(2, ℤ)) * h)⁻¹ • x = ((h⁻¹ : Λ) : SL(2, ℤ)) • (q.out⁻¹ : SL(2, ℤ)) • x := by
          rw [mul_inv_rev, mul_smul, Subgroup.coe_inv]
        rw [h1, hUinv _ (h⁻¹).2]
    _ = ∑' q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) *
          ∑' h : Λ, w (((h⁻¹ : Λ) : SL(2, ℤ)) • (q.out⁻¹ : SL(2, ℤ)) • x) :=
        tsum_congr fun q => ENNReal.tsum_mul_left
    _ = ∑' q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) * 2 := by
        refine tsum_congr fun q => ?_
        rw [← tsum_Λ_inv Λ (fun h => w ((h : SL(2, ℤ)) • (q.out⁻¹ : SL(2, ℤ)) • x)), hwt q]
    _ = 2 * ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) := by
        rw [ENNReal.tsum_mul_right, tsum_fintype, mul_comm]

theorem weight_lintegral (U w : ℍ → ℝ≥0∞) (hU : Measurable U) (hw : Measurable w)
    (hUinv : ∀ h ∈ Λ, ∀ x : ℍ, U (h • x) = U x)
    (hwt : ∀ᵐ x : ℍ, ∑' h : Λ, w ((h : SL(2, ℤ)) • x) = 2) :
    ∫⁻ x, U x * w x = ∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) := by
  have hm : Measurable fun x => U x * w x := hU.mul hw
  have hae : ∀ᵐ x : ℍ, ∀ q : SL(2, ℤ) ⧸ Λ,
      ∑' h : Λ, w ((h : SL(2, ℤ)) • (q.out⁻¹ : SL(2, ℤ)) • x) = 2 := by
    haveI : Countable (SL(2, ℤ) ⧸ Λ) := inferInstance
    rw [ae_all_iff]
    intro q
    exact (mp_smul_SL (q.out⁻¹ : SL(2, ℤ))).quasiMeasurePreserving.ae
      (p := fun y : ℍ => ∑' h : Λ, w ((h : SL(2, ℤ)) • y) = 2) hwt
  have hA : ∑' γ : SL(2, ℤ), ∫⁻ x in (𝒟 : Set ℍ), U (γ • x) * w (γ • x) = 2 * ∫⁻ z, U z * w z :=
    FLT.ModularFundamentalDomain.tsum_setLIntegral_fd_smul (fun x => U x * w x) hm
  have hB : ∑' γ : SL(2, ℤ), ∫⁻ x in (𝒟 : Set ℍ), U (γ • x) * w (γ • x)
      = ∫⁻ x in (𝒟 : Set ℍ), ∑' γ : SL(2, ℤ), U (γ • x) * w (γ • x) :=
    (lintegral_tsum fun γ =>
      ((hm.comp (measurable_smul_SL γ) : Measurable fun x => U (γ • x) * w (γ • x))).aemeasurable).symm
  have hC : ∫⁻ x in (𝒟 : Set ℍ), ∑' γ : SL(2, ℤ), U (γ • x) * w (γ • x)
      = ∫⁻ x in (𝒟 : Set ℍ), 2 * ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) := by
    refine lintegral_congr_ae (ae_restrict_of_ae ?_)
    filter_upwards [hae] with x hx
    exact tsum_G_eq Λ U w hUinv x hx
  have hmeasS : Measurable fun x : ℍ => ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) :=
    Finset.measurable_sum _ fun q _ =>
      (hU.comp (measurable_smul_SL _) : Measurable fun x => U ((q.out⁻¹ : SL(2, ℤ)) • x))
  have hD : ∫⁻ x in (𝒟 : Set ℍ), 2 * ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x)
      = 2 * ∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) :=
    lintegral_const_mul 2 hmeasS
  have key : (2 : ℝ≥0∞) * ∫⁻ x, U x * w x
      = 2 * ∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) := by
    rw [← hA, hB, hC, hD]
  exact (ENNReal.mul_right_inj two_ne_zero ENNReal.ofNat_ne_top).mp key

end weight

section cvn

variable (Λ₁ Λ₂ : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ₁)] [Fintype (SL(2, ℤ) ⧸ Λ₂)]

theorem cvn_lintegral (β : GL (Fin 2) ℝ) (c : Λ₁ ≃ Λ₂)
    (hc : ∀ h : Λ₁, β * (Matrix.SpecialLinearGroup.mapGL ℝ (h : SL(2, ℤ)) : GL (Fin 2) ℝ)
      = (Matrix.SpecialLinearGroup.mapGL ℝ ((c h : Λ₂) : SL(2, ℤ)) : GL (Fin 2) ℝ) * β)
    (U : ℍ → ℝ≥0∞) (hU : Measurable U) (hUinv : ∀ h ∈ Λ₂, ∀ x : ℍ, U (h • x) = U x) :
    ∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ₁, U (β • (q.out⁻¹ : SL(2, ℤ)) • x)
      = ∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ₂, U ((q.out⁻¹ : SL(2, ℤ)) • x) := by
  have hUβinv : ∀ h ∈ Λ₁, ∀ x : ℍ, U (β • h • x) = U (β • x) := by
    intro h hh x
    rw [hact, ← mul_smul, hc ⟨h, hh⟩, mul_smul, ← hact, hUinv _ (c ⟨h, hh⟩).2]
  have h1 := weight_lintegral Λ₁ (fun x => U (β • x)) (wt Λ₁) (hU.comp (measurable_smul_GL β))
    (measurable_wt Λ₁) hUβinv (ae_tsum_wt_smul Λ₁)
  have h2 : ∫⁻ x, U (β • x) * wt Λ₁ x = ∫⁻ y, U y * wt Λ₁ (β⁻¹ • y) := by
    have hmeas : Measurable fun y : ℍ => U y * wt Λ₁ (β⁻¹ • y) :=
      hU.mul ((measurable_wt Λ₁).comp (measurable_smul_GL β⁻¹))
    have := (mp_smul_GL β).lintegral_comp hmeas
    simpa only [inv_smul_smul] using this
  have hwt' : ∀ᵐ y : ℍ, ∑' h : Λ₂, wt Λ₁ (β⁻¹ • (h : SL(2, ℤ)) • y) = 2 := by
    have hT : ∀ᵐ y : ℍ, ∑' h : Λ₁, wt Λ₁ ((h : SL(2, ℤ)) • β⁻¹ • y) = 2 :=
      (mp_smul_GL β⁻¹).quasiMeasurePreserving.ae
        (p := fun z : ℍ => ∑' h : Λ₁, wt Λ₁ ((h : SL(2, ℤ)) • z) = 2) (ae_tsum_wt_smul Λ₁)
    filter_upwards [hT] with y hy
    rw [← hy, ← c.tsum_eq (fun h : Λ₂ => wt Λ₁ (β⁻¹ • (h : SL(2, ℤ)) • y))]
    refine tsum_congr fun h => ?_
    show wt Λ₁ (β⁻¹ • ((c h : Λ₂) : SL(2, ℤ)) • y) = wt Λ₁ ((h : SL(2, ℤ)) • β⁻¹ • y)
    congr 1
    rw [hact, hact, ← mul_smul, ← mul_smul]
    congr 1
    rw [eq_mul_inv_iff_mul_eq, mul_assoc, ← hc h, ← mul_assoc, inv_mul_cancel, one_mul]
  have h3 := weight_lintegral Λ₂ U (fun y => wt Λ₁ (β⁻¹ • y)) hU
    ((measurable_wt Λ₁).comp (measurable_smul_GL β⁻¹)) hUinv hwt'
  rw [← h3, ← h2, h1]

end cvn

section pairing

def dens (k : ℤ) (H : ℍ → ℂ) (x : ℍ) : ℝ := ‖H x‖ ^ 2 * x.im ^ k

theorem dens_nonneg (k : ℤ) (H : ℍ → ℂ) (x : ℍ) : 0 ≤ dens k H x :=
  mul_nonneg (sq_nonneg _) (zpow_nonneg x.im_pos.le _)

theorem petersson_self_eq_dens (k : ℤ) (H : ℍ → ℂ) (x : ℍ) :
    UpperHalfPlane.petersson k H H x = ((dens k H x : ℝ) : ℂ) := by
  unfold UpperHalfPlane.petersson dens
  rw [Complex.conj_mul' (H x)]
  push_cast
  ring

structure Nice (Λ : Subgroup SL(2, ℤ)) (k : ℤ) (H : ℍ → ℂ) : Prop where
  cont : Continuous H
  bdd : ∃ C, ∀ x, dens k H x ≤ C
  inv : ∀ h ∈ Λ, H ∣[k] (Matrix.SpecialLinearGroup.mapGL ℝ h : GL (Fin 2) ℝ) = H

variable {Λ : Subgroup SL(2, ℤ)} {k : ℤ}

theorem Nice.add {F G : ℍ → ℂ} (hF : Nice Λ k F) (hG : Nice Λ k G) : Nice Λ k (F + G) := by
  obtain ⟨C₁, hC₁⟩ := hF.bdd
  obtain ⟨C₂, hC₂⟩ := hG.bdd
  refine ⟨hF.cont.add hG.cont, ⟨2 * C₁ + 2 * C₂, fun x => ?_⟩, fun h hh => ?_⟩
  · have h1 := hC₁ x
    have h2 := hC₂ x
    unfold dens at h1 h2 ⊢
    have hy : 0 ≤ x.im ^ k := zpow_nonneg x.im_pos.le _
    have hn : ‖(F + G) x‖ ^ 2 ≤ 2 * ‖F x‖ ^ 2 + 2 * ‖G x‖ ^ 2 := by
      rw [Pi.add_apply]
      have h3 : ‖F x + G x‖ ^ 2 ≤ (‖F x‖ + ‖G x‖) ^ 2 :=
        pow_le_pow_left₀ (norm_nonneg _) (norm_add_le (F x) (G x)) 2
      nlinarith [h3, sq_nonneg (‖F x‖ - ‖G x‖)]
    nlinarith [mul_le_mul_of_nonneg_right hn hy]
  · rw [SlashAction.add_slash, hF.inv h hh, hG.inv h hh]

theorem Nice.smul {F : ℍ → ℂ} (hF : Nice Λ k F) (c : ℂ) : Nice Λ k (c • F) := by
  obtain ⟨C, hC⟩ := hF.bdd
  refine ⟨hF.cont.const_smul c, ⟨‖c‖ ^ 2 * C, fun x => ?_⟩, fun h hh => ?_⟩
  · have h1 := hC x
    unfold dens at h1 ⊢
    rw [Pi.smul_apply, smul_eq_mul, norm_mul, mul_pow, mul_assoc]
    exact mul_le_mul_of_nonneg_left h1 (sq_nonneg _)
  · have hσ : UpperHalfPlane.σ (Matrix.SpecialLinearGroup.mapGL ℝ h : GL (Fin 2) ℝ) = .refl ℝ ℂ := by
      rw [UpperHalfPlane.σ, if_pos (by simp)]
    rw [ModularForm.smul_slash, hF.inv h hh, hσ]
    rfl

theorem Nice.neg {F : ℍ → ℂ} (hF : Nice Λ k F) : Nice Λ k (-F) := by
  have := hF.smul (-1); rwa [neg_one_smul] at this

theorem Nice.sub {F G : ℍ → ℂ} (hF : Nice Λ k F) (hG : Nice Λ k G) : Nice Λ k (F - G) := by
  rw [sub_eq_add_neg]; exact hF.add hG.neg

theorem Nice.zero : Nice Λ k (0 : ℍ → ℂ) :=
  ⟨continuous_const, ⟨0, fun x => by simp [dens]⟩, fun _ _ => SlashAction.zero_slash k _⟩

theorem Nice.sum {ι : Type*} (s : Finset ι) {F : ι → ℍ → ℂ} (hF : ∀ i, Nice Λ k (F i)) :
    Nice Λ k (∑ i ∈ s, F i) := by
  classical
  induction s using Finset.induction_on with
  | empty => rw [Finset.sum_empty]; exact Nice.zero
  | insert a s ha ih => rw [Finset.sum_insert ha]; exact (hF a).add ih

theorem Nice.mono {Λ' : Subgroup SL(2, ℤ)} {F : ℍ → ℂ} (hF : Nice Λ k F) (h : Λ' ≤ Λ) : Nice Λ' k F :=
  ⟨hF.cont, hF.bdd, fun g hg => hF.inv g (h hg)⟩

theorem Nice.dens_smul {F : ℍ → ℂ} (hF : Nice Λ k F) {h : SL(2, ℤ)} (hh : h ∈ Λ) (x : ℍ) :
    dens k F (h • x) = dens k F x := by
  have := UpperHalfPlane.petersson_slash_SL k F F h x
  rw [ModularForm.SL_slash, show ((h : GL (Fin 2) ℝ)) = (Matrix.SpecialLinearGroup.mapGL ℝ h : GL (Fin 2) ℝ)
    from rfl, hF.inv h hh, petersson_self_eq_dens, petersson_self_eq_dens] at this
  exact_mod_cast this.symm

theorem continuous_dens {F : ℍ → ℂ} (hF : Continuous F) : Continuous (dens k F) := by
  unfold dens
  refine ((continuous_norm.comp hF).pow 2).mul ?_
  exact (UpperHalfPlane.continuous_im.zpow₀ k fun x => Or.inl (x.im_pos.ne'))

theorem norm_petersson_le {F G : ℍ → ℂ} (x : ℍ) :
    ‖UpperHalfPlane.petersson k F G x‖ ≤ (dens k F x + dens k G x) / 2 := by
  unfold UpperHalfPlane.petersson dens
  rw [norm_mul, norm_mul, Complex.norm_conj, Complex.norm_zpow, Complex.norm_real,
    Real.norm_of_nonneg x.im_pos.le]
  have hy : 0 ≤ x.im ^ k := zpow_nonneg x.im_pos.le _
  nlinarith [sq_nonneg (‖F x‖ - ‖G x‖), mul_nonneg (sq_nonneg (‖F x‖ - ‖G x‖)) hy]

variable (Λ) [Fintype (SL(2, ℤ) ⧸ Λ)]

def cosum (g : GL (Fin 2) ℝ) (Φ : ℍ → ℂ) (x : ℍ) : ℂ :=
  ∑ q : SL(2, ℤ) ⧸ Λ, Φ (g • (q.out⁻¹ : SL(2, ℤ)) • x)

def pair (k : ℤ) (F G : ℍ → ℂ) : ℂ :=
  ∫ x in (𝒟 : Set ℍ), cosum Λ 1 (UpperHalfPlane.petersson k F G) x

theorem integrable_cosum {F G : ℍ → ℂ} (hFc : Continuous F) (hGc : Continuous G)
    (hFb : ∃ C, ∀ x, dens k F x ≤ C) (hGb : ∃ C, ∀ x, dens k G x ≤ C) (g : GL (Fin 2) ℝ) :
    Integrable (cosum Λ g (UpperHalfPlane.petersson k F G)) (volume.restrict (𝒟 : Set ℍ)) := by
  obtain ⟨C₁, hC₁⟩ := hFb
  obtain ⟨C₂, hC₂⟩ := hGb
  have hmeas : Measurable (cosum Λ g (UpperHalfPlane.petersson k F G)) := by
    refine Finset.measurable_sum _ fun q _ => ?_
    exact (UpperHalfPlane.petersson_continuous k hFc hGc).measurable.comp
      ((measurable_smul_GL g).comp (measurable_smul_SL _))
  refine Measure.integrableOn_of_bounded (M := ∑ _q : SL(2, ℤ) ⧸ Λ, (C₁ + C₂) / 2)
    FLT.FundamentalDomainVolume.volume_fd_lt_top.ne hmeas.aestronglyMeasurable ?_
  refine Filter.Eventually.of_forall fun x => ?_
  refine (norm_sum_le _ _).trans (Finset.sum_le_sum fun q _ => ?_)
  refine (norm_petersson_le _).trans ?_
  gcongr
  · exact hC₁ _
  · exact hC₂ _

theorem integral_cosum_self {Λ₁ : Subgroup SL(2, ℤ)} [Fintype (SL(2, ℤ) ⧸ Λ₁)] (β : GL (Fin 2) ℝ)
    (c : Λ₁ ≃ Λ)
    (hc : ∀ h : Λ₁,
      β * (Matrix.SpecialLinearGroup.mapGL ℝ (h : SL(2, ℤ)) : GL (Fin 2) ℝ)
        = (Matrix.SpecialLinearGroup.mapGL ℝ ((c h : Λ) : SL(2, ℤ)) : GL (Fin 2) ℝ) * β)
    {H : ℍ → ℂ} (hH : Nice Λ k H) :
    ∫ x in (𝒟 : Set ℍ), cosum Λ₁ β (UpperHalfPlane.petersson k H H) x
      = ∫ x in (𝒟 : Set ℍ), cosum Λ 1 (UpperHalfPlane.petersson k H H) x := by
  have hre : ∀ (Λ' : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ')] (g : GL (Fin 2) ℝ),
      (cosum Λ' g (UpperHalfPlane.petersson k H H))
      = fun x => ((∑ q : SL(2, ℤ) ⧸ Λ', dens k H (g • (q.out⁻¹ : SL(2, ℤ)) • x) : ℝ) : ℂ) := by
    intro Λ' _ g; funext x
    simp only [cosum, petersson_self_eq_dens, Complex.ofReal_sum]
  have hmeas : ∀ (Λ' : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ')] (g : GL (Fin 2) ℝ),
      Measurable fun x : ℍ => ∑ q : SL(2, ℤ) ⧸ Λ', dens k H (g • (q.out⁻¹ : SL(2, ℤ)) • x) := by
    intro Λ' _ g
    refine Finset.measurable_sum _ fun q _ => ?_
    exact (continuous_dens hH.cont).measurable.comp ((measurable_smul_GL g).comp (measurable_smul_SL _))
  have hnn : ∀ (Λ' : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ')] (g : GL (Fin 2) ℝ) (x : ℍ),
      0 ≤ ∑ q : SL(2, ℤ) ⧸ Λ', dens k H (g • (q.out⁻¹ : SL(2, ℤ)) • x) :=
    fun Λ' _ g x => Finset.sum_nonneg fun q _ => dens_nonneg k H _
  have hred : ∀ (Λ' : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ')] (g : GL (Fin 2) ℝ),
      ∫ x in (𝒟 : Set ℍ), cosum Λ' g (UpperHalfPlane.petersson k H H) x
        = ((∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ',
            ENNReal.ofReal (dens k H (g • (q.out⁻¹ : SL(2, ℤ)) • x))).toReal : ℂ) := by
    intro Λ' _ g
    rw [hre Λ' g, integral_complex_ofReal, integral_eq_lintegral_of_nonneg_ae
      (Filter.Eventually.of_forall (hnn Λ' g)) (hmeas Λ' g).aestronglyMeasurable]
    congr 2
    refine lintegral_congr fun x => ?_
    rw [ENNReal.ofReal_sum_of_nonneg fun q _ => dens_nonneg k H _]
  rw [hred Λ₁ β, hred Λ 1]
  congr 2
  have hU : Measurable fun x => ENNReal.ofReal (dens k H x) :=
    ENNReal.measurable_ofReal.comp (continuous_dens hH.cont).measurable
  have := cvn_lintegral Λ₁ Λ β c hc
    (fun x => ENNReal.ofReal (dens k H x)) hU (fun h hh x => by simp only [hH.dens_smul hh])
  simpa only [one_smul] using this

theorem petersson_polarization (F G : ℍ → ℂ) (x : ℍ) :
    UpperHalfPlane.petersson k F G x
      = (4 : ℂ)⁻¹ * (UpperHalfPlane.petersson k (G + F) (G + F) x
          + Complex.I * UpperHalfPlane.petersson k (G + Complex.I • F) (G + Complex.I • F) x
          - UpperHalfPlane.petersson k (G - F) (G - F) x
          - Complex.I * UpperHalfPlane.petersson k (G - Complex.I • F) (G - Complex.I • F) x) := by
  simp only [UpperHalfPlane.petersson, Pi.add_apply, Pi.sub_apply, Pi.smul_apply, smul_eq_mul,
    map_add, map_sub, map_mul, Complex.conj_I]
  linear_combination (-(1 / 2 : ℂ) * ((starRingEnd ℂ) (G x) * F x - (starRingEnd ℂ) (F x) * G x)
    * ((x.im : ℂ)) ^ k) * Complex.I_sq

omit [Fintype (SL(2, ℤ) ⧸ Λ)] in

theorem integral_cosum_polar (Λ' : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ')]
    (g : GL (Fin 2) ℝ) {F G : ℍ → ℂ} (hF : Nice Λ k F) (hG : Nice Λ k G) :
    ∫ x in (𝒟 : Set ℍ), cosum Λ' g (UpperHalfPlane.petersson k F G) x
      = (4 : ℂ)⁻¹ * ((∫ x in (𝒟 : Set ℍ), cosum Λ' g (UpperHalfPlane.petersson k (G + F) (G + F)) x)
        + Complex.I * (∫ x in (𝒟 : Set ℍ), cosum Λ' g
            (UpperHalfPlane.petersson k (G + Complex.I • F) (G + Complex.I • F)) x)
        - (∫ x in (𝒟 : Set ℍ), cosum Λ' g (UpperHalfPlane.petersson k (G - F) (G - F)) x)
        - Complex.I * (∫ x in (𝒟 : Set ℍ), cosum Λ' g
            (UpperHalfPlane.petersson k (G - Complex.I • F) (G - Complex.I • F)) x)) := by
  have hexp : cosum Λ' g (UpperHalfPlane.petersson k F G)
      = fun x => (4 : ℂ)⁻¹ * (cosum Λ' g (UpperHalfPlane.petersson k (G + F) (G + F)) x
          + Complex.I * cosum Λ' g (UpperHalfPlane.petersson k (G + Complex.I • F) (G + Complex.I • F)) x
          - cosum Λ' g (UpperHalfPlane.petersson k (G - F) (G - F)) x
          - Complex.I * cosum Λ' g (UpperHalfPlane.petersson k (G - Complex.I • F) (G - Complex.I • F)) x) := by
    funext x
    simp only [cosum, Finset.mul_sum, ← Finset.sum_add_distrib, ← Finset.sum_sub_distrib]
    exact Finset.sum_congr rfl fun q _ => by rw [petersson_polarization]
  have h0 := hG.add hF
  have h1 := hG.add (hF.smul Complex.I)
  have h2 := hG.sub hF
  have h3 := hG.sub (hF.smul Complex.I)
  have iA := integrable_cosum Λ' h0.cont h0.cont h0.bdd h0.bdd g
  have iB := (integrable_cosum Λ' h1.cont h1.cont h1.bdd h1.bdd g).const_mul Complex.I
  have iC := integrable_cosum Λ' h2.cont h2.cont h2.bdd h2.bdd g
  have iD := (integrable_cosum Λ' h3.cont h3.cont h3.bdd h3.bdd g).const_mul Complex.I
  have iAB : Integrable (fun x => cosum Λ' g (UpperHalfPlane.petersson k (G + F) (G + F)) x
      + Complex.I * cosum Λ' g (UpperHalfPlane.petersson k (G + Complex.I • F) (G + Complex.I • F)) x)
      (volume.restrict (𝒟 : Set ℍ)) := iA.add iB
  have iABC : Integrable (fun x => cosum Λ' g (UpperHalfPlane.petersson k (G + F) (G + F)) x
      + Complex.I * cosum Λ' g (UpperHalfPlane.petersson k (G + Complex.I • F) (G + Complex.I • F)) x
      - cosum Λ' g (UpperHalfPlane.petersson k (G - F) (G - F)) x)
      (volume.restrict (𝒟 : Set ℍ)) := iAB.sub iC
  rw [hexp, integral_const_mul]
  congr 1
  rw [integral_sub iABC iD, integral_sub iAB iC, integral_add iA iB, integral_const_mul,
    integral_const_mul]

theorem integral_cosum_pair {Λ₁ : Subgroup SL(2, ℤ)} [Fintype (SL(2, ℤ) ⧸ Λ₁)] (β : GL (Fin 2) ℝ)
    (c : Λ₁ ≃ Λ)
    (hc : ∀ h : Λ₁,
      β * (Matrix.SpecialLinearGroup.mapGL ℝ (h : SL(2, ℤ)) : GL (Fin 2) ℝ)
        = (Matrix.SpecialLinearGroup.mapGL ℝ ((c h : Λ) : SL(2, ℤ)) : GL (Fin 2) ℝ) * β)
    {F G : ℍ → ℂ} (hF : Nice Λ k F) (hG : Nice Λ k G) :
    ∫ x in (𝒟 : Set ℍ), cosum Λ₁ β (UpperHalfPlane.petersson k F G) x
      = ∫ x in (𝒟 : Set ℍ), cosum Λ 1 (UpperHalfPlane.petersson k F G) x := by
  rw [integral_cosum_polar Λ Λ₁ β hF hG, integral_cosum_polar Λ Λ 1 hF hG,
    integral_cosum_self Λ β c hc (hG.add hF), integral_cosum_self Λ β c hc (hG.add (hF.smul _)),
    integral_cosum_self Λ β c hc (hG.sub hF), integral_cosum_self Λ β c hc (hG.sub (hF.smul _))]

theorem pair_slash {Λ₁ : Subgroup SL(2, ℤ)} [Fintype (SL(2, ℤ) ⧸ Λ₁)] (β : GL (Fin 2) ℝ)
    (hβ : 0 < (β.det : ℝ)) (c : Λ₁ ≃ Λ)
    (hc : ∀ h : Λ₁,
      β * (Matrix.SpecialLinearGroup.mapGL ℝ (h : SL(2, ℤ)) : GL (Fin 2) ℝ)
        = (Matrix.SpecialLinearGroup.mapGL ℝ ((c h : Λ) : SL(2, ℤ)) : GL (Fin 2) ℝ) * β)
    {F G : ℍ → ℂ} (hF : Nice Λ k F) (hG : Nice Λ k G) :
    pair Λ₁ k (F ∣[k] β) (G ∣[k] β) = ((β.det : ℝ) : ℂ) ^ (k - 2) * pair Λ k F G := by
  unfold pair
  have hσ : UpperHalfPlane.σ β = .refl ℝ ℂ := by rw [UpperHalfPlane.σ, if_pos hβ]
  have hpt : cosum Λ₁ 1 (UpperHalfPlane.petersson k (F ∣[k] β) (G ∣[k] β))
      = fun x => ((β.det : ℝ) : ℂ) ^ (k - 2) * cosum Λ₁ β (UpperHalfPlane.petersson k F G) x := by
    funext x
    simp only [cosum, one_smul, Finset.mul_sum]
    refine Finset.sum_congr rfl fun q _ => ?_
    rw [UpperHalfPlane.petersson_slash, hσ, abs_of_pos hβ]
    rfl
  rw [hpt, integral_const_mul, integral_cosum_pair Λ β c hc hF hG]

theorem pair_conj_symm (F G : ℍ → ℂ) : pair Λ k G F = conj (pair Λ k F G) := by
  unfold pair
  rw [← integral_conj]
  congr 1
  funext x
  simp only [cosum, map_sum]
  exact Finset.sum_congr rfl fun q _ => UpperHalfPlane.petersson_symm k F G _

theorem pair_smul_left (c : ℂ) (F G : ℍ → ℂ) : pair Λ k (c • F) G = conj c * pair Λ k F G := by
  unfold pair
  rw [← integral_const_mul]
  congr 1; funext x
  simp only [cosum, Finset.mul_sum]
  refine Finset.sum_congr rfl fun q _ => ?_
  simp only [UpperHalfPlane.petersson, Pi.smul_apply, smul_eq_mul, map_mul]; ring

theorem pair_smul_right (c : ℂ) (F G : ℍ → ℂ) : pair Λ k F (c • G) = c * pair Λ k F G := by
  unfold pair
  rw [← integral_const_mul]
  congr 1; funext x
  simp only [cosum, Finset.mul_sum]
  refine Finset.sum_congr rfl fun q _ => ?_
  simp only [UpperHalfPlane.petersson, Pi.smul_apply, smul_eq_mul]; ring

theorem pair_sum_left {ι : Type*} (s : Finset ι) (F : ι → ℍ → ℂ) (G : ℍ → ℂ)
    (hF : ∀ i, Nice Λ k (F i)) (hG : Nice Λ k G) :
    pair Λ k (∑ i ∈ s, F i) G = ∑ i ∈ s, pair Λ k (F i) G := by
  classical
  induction s using Finset.induction_on with
  | empty =>
    simp only [Finset.sum_empty, pair, cosum]
    simp [UpperHalfPlane.petersson]
  | insert a s ha ih =>
    rw [Finset.sum_insert ha, Finset.sum_insert ha, ← ih]
    unfold pair
    rw [← integral_add (integrable_cosum Λ (hF a).cont hG.cont (hF a).bdd hG.bdd 1)]
    · congr 1; funext x
      simp only [cosum, ← Finset.sum_add_distrib]
      refine Finset.sum_congr rfl fun q _ => ?_
      simp only [UpperHalfPlane.petersson, Pi.add_apply, Finset.sum_apply, map_add]; ring
    · have hs : Nice Λ k (∑ i ∈ s, F i) := Nice.sum s hF
      exact integrable_cosum Λ hs.cont hG.cont hs.bdd hG.bdd 1

theorem pair_sum_right {ι : Type*} (s : Finset ι) (F : ℍ → ℂ) (G : ι → ℍ → ℂ)
    (hF : Nice Λ k F) (hG : ∀ i, Nice Λ k (G i)) :
    pair Λ k F (∑ i ∈ s, G i) = ∑ i ∈ s, pair Λ k F (G i) := by
  rw [pair_conj_symm, pair_sum_left Λ s G F hG hF, map_sum]
  exact Finset.sum_congr rfl fun i _ => by rw [pair_conj_symm Λ (k := k) F (G i), Complex.conj_conj]

end pairing

section definite

variable (Λ : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ)] {k : ℤ}

theorem pair_self_eq_lintegral {F : ℍ → ℂ} (hF : Continuous F) :
    pair Λ k F F = ((∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ,
        ENNReal.ofReal (dens k F ((q.out⁻¹ : SL(2, ℤ)) • x))).toReal : ℂ) := by
  unfold pair
  have hre : cosum Λ 1 (UpperHalfPlane.petersson k F F)
      = fun x => ((∑ q : SL(2, ℤ) ⧸ Λ, dens k F ((q.out⁻¹ : SL(2, ℤ)) • x) : ℝ) : ℂ) := by
    funext x
    simp only [cosum, one_smul, petersson_self_eq_dens, Complex.ofReal_sum]
  have hmeas : Measurable fun x : ℍ => ∑ q : SL(2, ℤ) ⧸ Λ, dens k F ((q.out⁻¹ : SL(2, ℤ)) • x) := by
    refine Finset.measurable_sum _ fun q _ => ?_
    exact (continuous_dens hF).measurable.comp (measurable_smul_SL _)
  rw [hre, integral_complex_ofReal, integral_eq_lintegral_of_nonneg_ae
    (Filter.Eventually.of_forall fun x => Finset.sum_nonneg fun q _ => dens_nonneg k F _)
    hmeas.aestronglyMeasurable]
  congr 2
  refine lintegral_congr fun x => ?_
  rw [ENNReal.ofReal_sum_of_nonneg fun q _ => dens_nonneg k F _]

theorem eq_zero_of_pair_self_eq_zero {F : ℍ → ℂ} (hF : Nice Λ k F) (h0 : pair Λ k F F = 0) :
    F = 0 := by
  obtain ⟨C, hC⟩ := hF.bdd
  set U : ℍ → ℝ≥0∞ := fun x => ENNReal.ofReal (dens k F x) with hUdef
  have hU : Measurable U := ENNReal.measurable_ofReal.comp (continuous_dens hF.cont).measurable
  have hUinv : ∀ h ∈ Λ, ∀ x, U (h • x) = U x := fun h hh x => by simp only [hUdef, hF.dens_smul hh]

  set I : ℝ≥0∞ := ∫⁻ x in (𝒟 : Set ℍ), ∑ q : SL(2, ℤ) ⧸ Λ, U ((q.out⁻¹ : SL(2, ℤ)) • x) with hI
  have hIfin : I ≠ ⊤ := by
    refine ne_top_of_le_ne_top ?_ (lintegral_mono (g := fun _ => ∑ _q : SL(2, ℤ) ⧸ Λ, ENNReal.ofReal C)
      fun x => Finset.sum_le_sum fun q _ => ENNReal.ofReal_le_ofReal (hC _))
    rw [lintegral_const]
    refine ENNReal.mul_ne_top ?_ (by
      rw [Measure.restrict_apply_univ]; exact FLT.FundamentalDomainVolume.volume_fd_lt_top.ne)
    simp only [Finset.sum_const, Finset.card_univ, nsmul_eq_mul]
    exact ENNReal.mul_ne_top (ENNReal.natCast_ne_top _) ENNReal.ofReal_ne_top
  have hI0 : I = 0 := by
    have h := pair_self_eq_lintegral Λ (k := k) hF.cont
    rw [h0] at h
    have h' : I.toReal = 0 := by exact_mod_cast h.symm
    rcases (ENNReal.toReal_eq_zero_iff _).mp h' with h'' | h''
    · exact h''
    · exact absurd h'' hIfin

  have hterm : ∀ q : SL(2, ℤ) ⧸ Λ, ∫⁻ x in (𝒟 : Set ℍ), U ((q.out⁻¹ : SL(2, ℤ)) • x) = 0 := by
    have hsum : ∑ q : SL(2, ℤ) ⧸ Λ, ∫⁻ x in (𝒟 : Set ℍ), U ((q.out⁻¹ : SL(2, ℤ)) • x) = 0 := by
      have h := lintegral_finsetSum (μ := volume.restrict (𝒟 : Set ℍ)) Finset.univ
        (f := fun (q : SL(2, ℤ) ⧸ Λ) (x : ℍ) => U ((q.out⁻¹ : SL(2, ℤ)) • x))
        (fun q _ => (hU.comp (measurable_smul_SL _)))
      rw [← h]
      exact hI0
    intro q
    exact Finset.sum_eq_zero_iff.mp hsum q (Finset.mem_univ q)

  have hall : ∀ σ : SL(2, ℤ), ∫⁻ x in (𝒟 : Set ℍ), U (σ • x) = 0 := by
    intro σ
    obtain ⟨h, hh⟩ := QuotientGroup.mk_out_eq_mul Λ σ⁻¹
    set q : SL(2, ℤ) ⧸ Λ := QuotientGroup.mk σ⁻¹ with hq
    have hσ : σ = (h : SL(2, ℤ)) * (q.out)⁻¹ := by
      rw [hh, mul_inv_rev, inv_inv, ← mul_assoc, mul_inv_cancel, one_mul]
    have : (fun x => U (σ • x)) = fun x => U ((q.out)⁻¹ • x) := by
      funext x
      rw [hσ, mul_smul, hUinv _ h.2]
    rw [this]
    exact hterm q

  have htot : ∫⁻ x, U x = 0 := by
    have h := FLT.ModularFundamentalDomain.tsum_setLIntegral_fd_smul U hU
    simp only [hall, tsum_zero] at h
    have h2 : (2 : ℝ≥0∞) * ∫⁻ x, U x = 0 := h.symm
    simpa using h2
  have hae : U =ᵐ[volume] 0 := (lintegral_eq_zero_iff hU).mp htot
  have hdens : dens k F = fun _ => 0 := by
    have hae' : dens k F =ᵐ[volume] fun _ => (0 : ℝ) := by
      filter_upwards [hae] with x hx
      have hx' : ENNReal.ofReal (dens k F x) = 0 := hx
      exact le_antisymm (ENNReal.ofReal_eq_zero.mp hx') (dens_nonneg k F x)
    exact (Continuous.ae_eq_iff_eq volume (continuous_dens hF.cont) continuous_const).mp hae'
  funext x
  have hx := congrFun hdens x
  unfold dens at hx
  have hy : 0 < x.im ^ k := zpow_pos x.im_pos _
  have : ‖F x‖ ^ 2 = 0 := by
    rcases mul_eq_zero.mp hx with h | h
    · exact h
    · exact absurd h hy.ne'
  simpa using this

end definite

section level

variable {Γ'' Λ : Subgroup SL(2, ℤ)}

section unfold

variable (hΛΓ : Λ ≤ Γ'') {ι : Type*} (r : ι → SL(2, ℤ)) (hr : ∀ i, r i ∈ Γ'')
  (hex : ∀ γ ∈ Γ'', ∃ i, r i * γ ∈ Λ) (huniq : ∀ i i', r i * (r i')⁻¹ ∈ Λ → i = i')

def eMap (r : ι → SL(2, ℤ)) (x : (SL(2, ℤ) ⧸ Γ'') × ι) : SL(2, ℤ) ⧸ Λ :=
  QuotientGroup.mk (x.1.out * (r x.2)⁻¹)

include hΛΓ hr hex huniq in
theorem eMap_bijective : Function.Bijective (eMap (Γ'' := Γ'') (Λ := Λ) r) := by
  constructor
  · rintro ⟨q, i⟩ ⟨q', i'⟩ h
    have hmem : r i * q.out⁻¹ * q'.out * (r i')⁻¹ ∈ Λ := by
      have := QuotientGroup.eq.mp h
      simpa [mul_assoc, mul_inv_rev] using this
    have hqq : q = q' := by
      have hΓ : q.out⁻¹ * q'.out ∈ Γ'' := by
        have h1 : (r i)⁻¹ * (r i * q.out⁻¹ * q'.out * (r i')⁻¹) * r i' ∈ Γ'' :=
          mul_mem (mul_mem (inv_mem (hr i)) (hΛΓ hmem)) (hr i')
        simpa [mul_assoc] using h1
      rw [← QuotientGroup.out_eq' q, ← QuotientGroup.out_eq' q']
      exact QuotientGroup.eq.mpr hΓ
    subst hqq
    have : r i * (r i')⁻¹ ∈ Λ := by simpa using hmem
    rw [huniq i i' this]
  · intro y
    set σ := y.out
    set q : SL(2, ℤ) ⧸ Γ'' := QuotientGroup.mk σ
    have hγ : q.out⁻¹ * σ ∈ Γ'' := by
      rw [← QuotientGroup.eq]
      exact QuotientGroup.out_eq' q
    obtain ⟨i, hi⟩ := hex _ hγ
    refine ⟨(q, i), ?_⟩
    rw [← QuotientGroup.out_eq' y]
    show QuotientGroup.mk (q.out * (r i)⁻¹) = QuotientGroup.mk σ
    rw [QuotientGroup.eq]
    simpa [mul_assoc, mul_inv_rev] using hi

variable [Fintype (SL(2, ℤ) ⧸ Γ'')] [Fintype (SL(2, ℤ) ⧸ Λ)] [Fintype ι]

include hΛΓ hr hex huniq in

theorem unfold_sum {X : Type*} [AddCommMonoid X] (Φ : ℍ → X)
    (hΦ : ∀ h ∈ Λ, ∀ x : ℍ, Φ (h • x) = Φ x) (τ : ℍ) :
    ∑ q' : SL(2, ℤ) ⧸ Λ, Φ ((q'.out⁻¹ : SL(2, ℤ)) • τ)
      = ∑ q : SL(2, ℤ) ⧸ Γ'', ∑ i, Φ ((r i * q.out⁻¹ : SL(2, ℤ)) • τ) := by
  rw [← Fintype.sum_prod_type']
  symm
  refine Fintype.sum_bijective (eMap (Γ'' := Γ'') (Λ := Λ) r)
    (eMap_bijective hΛΓ r hr hex huniq) _ _ fun x => ?_
  obtain ⟨h, hh⟩ := QuotientGroup.mk_out_eq_mul Λ (x.1.out * (r x.2)⁻¹)
  show Φ ((r x.2 * x.1.out⁻¹ : SL(2, ℤ)) • τ) = Φ (((eMap r x).out⁻¹ : SL(2, ℤ)) • τ)
  rw [eMap, hh, mul_inv_rev, mul_inv_rev, inv_inv, mul_smul ((h : SL(2, ℤ))⁻¹) (r x.2 * x.1.out⁻¹) τ,
    hΦ _ (inv_mem h.2)]

end unfold

variable [Fintype (SL(2, ℤ) ⧸ Γ'')] [Fintype (SL(2, ℤ) ⧸ Λ)]

theorem cosum_eq_relIndex_mul (hle : Λ ≤ Γ'') (Φ : ℍ → ℂ)
    (hΦ : ∀ h ∈ Γ'', ∀ x : ℍ, Φ (h • x) = Φ x) (τ : ℍ) :
    cosum Λ 1 Φ τ = (Λ.relIndex Γ'' : ℂ) * cosum Γ'' 1 Φ τ := by

  have hne : (Λ.subgroupOf Γ'').index ≠ 0 := by
    have h := Subgroup.relIndex_mul_index hle
    intro h0
    rw [Subgroup.relIndex, h0, zero_mul] at h
    have hΛ : Λ.index ≠ 0 := by
      rw [Subgroup.index_eq_card, Nat.card_eq_fintype_card]; exact Fintype.card_ne_zero
    exact hΛ h.symm
  haveI : (Λ.subgroupOf Γ'').FiniteIndex := ⟨hne⟩
  haveI : Finite (Γ'' ⧸ Λ.subgroupOf Γ'') := Subgroup.finite_quotient_of_finiteIndex
  letI : Fintype (Γ'' ⧸ Λ.subgroupOf Γ'') := Fintype.ofFinite _
  set r : (Γ'' ⧸ Λ.subgroupOf Γ'') → SL(2, ℤ) := fun i => ((i.out : Γ'') : SL(2, ℤ))⁻¹ with hr
  have hr' : ∀ i, r i ∈ Γ'' := fun i => inv_mem (i.out).2
  have hex : ∀ γ ∈ Γ'', ∃ i, r i * γ ∈ Λ := by
    intro γ hγ
    refine ⟨QuotientGroup.mk ⟨γ, hγ⟩, ?_⟩
    have h := QuotientGroup.out_eq' (QuotientGroup.mk (s := Λ.subgroupOf Γ'') ⟨γ, hγ⟩)
    have h2 := QuotientGroup.eq.mp h
    rw [Subgroup.mem_subgroupOf] at h2
    simpa [hr] using h2
  have huniq : ∀ i i', r i * (r i')⁻¹ ∈ Λ → i = i' := by
    intro i i' h
    have h2 : (i.out)⁻¹ * i'.out ∈ Λ.subgroupOf Γ'' := by
      rw [Subgroup.mem_subgroupOf]; simpa [hr] using h
    have := QuotientGroup.eq.mpr h2
    rwa [QuotientGroup.out_eq', QuotientGroup.out_eq'] at this
  unfold cosum
  simp only [one_smul]
  rw [unfold_sum hle r hr' hex huniq Φ (fun h hh x => hΦ h (hle hh) x) τ, Finset.mul_sum]
  refine Finset.sum_congr rfl fun q _ => ?_
  have : ∀ i, Φ ((r i * q.out⁻¹ : SL(2, ℤ)) • τ) = Φ ((q.out⁻¹ : SL(2, ℤ)) • τ) := fun i => by
    rw [mul_smul, hΦ _ (hr' i)]
  simp only [this, Finset.sum_const, Finset.card_univ, nsmul_eq_mul]
  congr 1
  rw [Subgroup.relIndex, Subgroup.index_eq_card, Nat.card_eq_fintype_card]

theorem pair_eq_relIndex_mul (hle : Λ ≤ Γ'') {k : ℤ} {F G : ℍ → ℂ} (hF : Nice Γ'' k F) (hG : Nice Γ'' k G) :
    pair Λ k F G = (Λ.relIndex Γ'' : ℂ) * pair Γ'' k F G := by
  unfold pair
  rw [← integral_const_mul]
  congr 1
  funext τ
  refine cosum_eq_relIndex_mul hle _ (fun h hh x => ?_) τ
  have := UpperHalfPlane.petersson_slash_SL k F G h x
  rw [ModularForm.SL_slash, ModularForm.SL_slash,
    show ((h : GL (Fin 2) ℝ)) = (Matrix.SpecialLinearGroup.mapGL ℝ h : GL (Fin 2) ℝ) from rfl,
    hF.inv h hh, hG.inv h hh] at this
  exact this.symm

end level

section indexeq

variable (Λ₁ Λ₂ : Subgroup SL(2, ℤ)) [Fintype (SL(2, ℤ) ⧸ Λ₁)] [Fintype (SL(2, ℤ) ⧸ Λ₂)]

theorem card_quotient_eq_of_conj (β : GL (Fin 2) ℝ) (c : Λ₁ ≃ Λ₂)
    (hc : ∀ h : Λ₁, β * (Matrix.SpecialLinearGroup.mapGL ℝ (h : SL(2, ℤ)) : GL (Fin 2) ℝ)
      = (Matrix.SpecialLinearGroup.mapGL ℝ ((c h : Λ₂) : SL(2, ℤ)) : GL (Fin 2) ℝ) * β) :
    Fintype.card (SL(2, ℤ) ⧸ Λ₁) = Fintype.card (SL(2, ℤ) ⧸ Λ₂) := by
  have h := cvn_lintegral Λ₁ Λ₂ β c hc (fun _ => 1) measurable_const (fun _ _ _ => rfl)
  simp only [Finset.sum_const, Finset.card_univ, nsmul_eq_mul, mul_one, lintegral_const,
    Measure.restrict_apply_univ] at h
  obtain ⟨h0, htop⟩ := FLT.FundamentalDomainVolume.volume_fd_ne_zero_ne_top
  have := (ENNReal.mul_left_inj h0 htop).mp h
  exact_mod_cast this

end indexeq

end CuspForm.PeterssonCoset

end