Module: Mathlib.NumberTheory.NumberField.Discriminant.Different

Goal state before:
pp: K : Type u_1
𝒪 : Type u_2
inst✝⁷ : Field K
inst✝⁶ : NumberField K
inst✝⁵ : CommRing 𝒪
inst✝⁴ : Algebra 𝒪 K
inst✝³ : IsFractionRing 𝒪 K
inst✝² : IsDedekindDomain 𝒪
inst✝¹ : CharZero 𝒪
inst✝ : Module.Finite ℤ 𝒪
this✝ :
  (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) ≃ₗ[𝒪]
    ↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1
this :
  Nat.card (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) =
    Nat.card (↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1)
b : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := integralBasis K
b' : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := (Algebra.traceForm ℚ K).dualBasis ⋯ b
hb : Submodule.span ℤ (Set.range ⇑b) = Submodule.restrictScalars ℤ 1
⊢ ↑(Nat.card (↥↑(FractionalIdeal.dual ℤ ℚ 1) ⧸ Submodule.comap (↑(FractionalIdeal.dual ℤ ℚ 1)).subtype 1)) =
    |↑(discr K)|

Tactic:
refine (AddSubgroup.relIndex_eq_abs_det (1 : Submodule 𝒪 K).toAddSubgroup (FractionalIdeal.dual
    ℤ ℚ 1 : FractionalIdeal 𝒪⁰ K).coeToSubmodule.toAddSubgroup ?_ b b' ?_ ?_).trans ?_

Goal state after:
case refine_1
K : Type u_1
𝒪 : Type u_2
inst✝⁷ : Field K
inst✝⁶ : NumberField K
inst✝⁵ : CommRing 𝒪
inst✝⁴ : Algebra 𝒪 K
inst✝³ : IsFractionRing 𝒪 K
inst✝² : IsDedekindDomain 𝒪
inst✝¹ : CharZero 𝒪
inst✝ : Module.Finite ℤ 𝒪
this✝ :
  (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) ≃ₗ[𝒪]
    ↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1
this :
  Nat.card (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) =
    Nat.card (↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1)
b : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := integralBasis K
b' : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := (Algebra.traceForm ℚ K).dualBasis ⋯ b
hb : Submodule.span ℤ (Set.range ⇑b) = Submodule.restrictScalars ℤ 1
⊢ Submodule.toAddSubgroup 1 ≤ (↑(FractionalIdeal.dual ℤ ℚ 1)).toAddSubgroup

case refine_2
K : Type u_1
𝒪 : Type u_2
inst✝⁷ : Field K
inst✝⁶ : NumberField K
inst✝⁵ : CommRing 𝒪
inst✝⁴ : Algebra 𝒪 K
inst✝³ : IsFractionRing 𝒪 K
inst✝² : IsDedekindDomain 𝒪
inst✝¹ : CharZero 𝒪
inst✝ : Module.Finite ℤ 𝒪
this✝ :
  (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) ≃ₗ[𝒪]
    ↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1
this :
  Nat.card (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) =
    Nat.card (↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1)
b : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := integralBasis K
b' : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := (Algebra.traceForm ℚ K).dualBasis ⋯ b
hb : Submodule.span ℤ (Set.range ⇑b) = Submodule.restrictScalars ℤ 1
⊢ Submodule.toAddSubgroup 1 = AddSubgroup.closure (Set.range ⇑b)

case refine_3
K : Type u_1
𝒪 : Type u_2
inst✝⁷ : Field K
inst✝⁶ : NumberField K
inst✝⁵ : CommRing 𝒪
inst✝⁴ : Algebra 𝒪 K
inst✝³ : IsFractionRing 𝒪 K
inst✝² : IsDedekindDomain 𝒪
inst✝¹ : CharZero 𝒪
inst✝ : Module.Finite ℤ 𝒪
this✝ :
  (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) ≃ₗ[𝒪]
    ↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1
this :
  Nat.card (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) =
    Nat.card (↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1)
b : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := integralBasis K
b' : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := (Algebra.traceForm ℚ K).dualBasis ⋯ b
hb : Submodule.span ℤ (Set.range ⇑b) = Submodule.restrictScalars ℤ 1
⊢ (↑(FractionalIdeal.dual ℤ ℚ 1)).toAddSubgroup = AddSubgroup.closure (Set.range ⇑b')

case refine_4
K : Type u_1
𝒪 : Type u_2
inst✝⁷ : Field K
inst✝⁶ : NumberField K
inst✝⁵ : CommRing 𝒪
inst✝⁴ : Algebra 𝒪 K
inst✝³ : IsFractionRing 𝒪 K
inst✝² : IsDedekindDomain 𝒪
inst✝¹ : CharZero 𝒪
inst✝ : Module.Finite ℤ 𝒪
this✝ :
  (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) ≃ₗ[𝒪]
    ↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1
this :
  Nat.card (↥↑1 ⧸ Submodule.comap (↑1).subtype ↑↑(differentIdeal ℤ 𝒪)) =
    Nat.card (↥↑(↑(differentIdeal ℤ 𝒪))⁻¹ ⧸ Submodule.comap (↑(↑(differentIdeal ℤ 𝒪))⁻¹).subtype ↑1)
b : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := integralBasis K
b' : Basis (Free.ChooseBasisIndex ℤ (𝓞 K)) ℚ K := (Algebra.traceForm ℚ K).dualBasis ⋯ b
hb : Submodule.span ℤ (Set.range ⇑b) = Submodule.restrictScalars ℤ 1
⊢ |b'.det ⇑b| = |↑(discr K)|