Module: Mathlib.NumberTheory.NumberField.Discriminant.Different

Goal state before:
pp: 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

pp: 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)

pp: 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')

pp: 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)|

Tactic:
· rw [Submodule.toAddSubgroup_le, ← FractionalIdeal.coe_one]
    exact FractionalIdeal.one_le_dual_one ℤ ℚ (L := K) (B := 𝒪)

Goal state after:
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)|