-- Mathlib/NumberTheory/NumberField/Discriminant/Defs.lean
theorem Algebra.discr_eq_discr_of_toMatrix_coeff_isIntegral [NumberField K]
    {b : Basis ι ℚ K} {b' : Basis ι' ℚ K} (h : ∀ i j, IsIntegral ℤ (b.toMatrix b' i j))
    (h' : ∀ i j, IsIntegral ℤ (b'.toMatrix b i j)) : discr ℚ b = discr ℚ b'

Proof state:
case e_a.intro.inl
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
r : ℤ
hr : (algebraMap ℤ ℚ) r = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
hunit : IsUnit r
hp : r = 1
⊢ (algebraMap ℤ ℚ) 1 = (algebraMap ℤ ℚ) r ^ 2

Tactic:
simp [hp]

Result:
no goals

Proof state:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i : ι') (j : ι), IsIntegral ℤ (b'.toMatrix (⇑b) i j)
⊢ ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)

Tactic:
intro i j

Result:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i : ι') (j : ι), IsIntegral ℤ (b'.toMatrix (⇑b) i j)
i j : ι'
⊢ IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)

Proof state:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
⊢ 1 * discr ℚ ⇑b = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2 * discr ℚ ⇑b

Tactic:
congr

Result:
case e_a
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
⊢ 1 = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2

Proof state:
case e_a.intro
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
r : ℤ
hr : (algebraMap ℤ ℚ) r = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
⊢ 1 = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2

Tactic:
have hunit : IsUnit r := by
  have : IsIntegral ℤ (b'.toMatrix (b.reindex (b.indexEquiv b'))).det :=
    IsIntegral.det fun i j => h' _ _
  obtain ⟨r', hr'⟩ := IsIntegrallyClosed.isIntegral_iff.1 this
  refine isUnit_iff_exists_inv.2 ⟨r', ?_⟩
  suffices algebraMap ℤ ℚ (r * r') = 1 by
    rw [← RingHom.map_one (algebraMap ℤ ℚ)] at this
    exact (IsFractionRing.injective ℤ ℚ) this
  rw [RingHom.map_mul, hr, hr', ← Matrix.det_mul,
    Basis.toMatrix_mul_toMatrix_flip, Matrix.det_one]

Result:
case e_a.intro
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
r : ℤ
hr : (algebraMap ℤ ℚ) r = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
hunit : IsUnit r
⊢ 1 = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2

Proof state:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
r : ℤ
hr : (algebraMap ℤ ℚ) r = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
this✝ : IsIntegral ℤ (b'.toMatrix ⇑(b.reindex (b.indexEquiv b'))).det
r' : ℤ
hr' : (algebraMap ℤ ℚ) r' = (b'.toMatrix ⇑(b.reindex (b.indexEquiv b'))).det
this : (algebraMap ℤ ℚ) (r * r') = 1
⊢ r * r' = 1

Tactic:
rw [← RingHom.map_one (algebraMap ℤ ℚ)] at this

Result:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
r : ℤ
hr : (algebraMap ℤ ℚ) r = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
this✝ : IsIntegral ℤ (b'.toMatrix ⇑(b.reindex (b.indexEquiv b'))).det
r' : ℤ
hr' : (algebraMap ℤ ℚ) r' = (b'.toMatrix ⇑(b.reindex (b.indexEquiv b'))).det
this : (algebraMap ℤ ℚ) (r * r') = (algebraMap ℤ ℚ) 1
⊢ r * r' = 1

Proof state:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
⊢ discr ℚ ⇑b = discr ℚ ⇑b'

Tactic:
rw [← (b.reindex (b.indexEquiv b')).toMatrix_map_vecMul b', discr_of_matrix_vecMul,
  ← one_mul (discr ℚ b), Basis.coe_reindex, discr_reindex]

Result:
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
⊢ 1 * discr ℚ ⇑b = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2 * discr ℚ ⇑b

Proof state:
case e_a
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
⊢ 1 = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2

Tactic:
obtain ⟨r, hr⟩ := IsIntegrallyClosed.isIntegral_iff.1 hint

Result:
case e_a.intro
ι : Type u_2
ι' : Type u_3
K : Type u_1
inst✝⁵ : Field K
inst✝⁴ : DecidableEq ι
inst✝³ : DecidableEq ι'
inst✝² : Fintype ι
inst✝¹ : Fintype ι'
inst✝ : NumberField K
b : Basis ι ℚ K
b' : Basis ι' ℚ K
h : ∀ (i : ι) (j : ι'), IsIntegral ℤ (b.toMatrix (⇑b') i j)
h' : ∀ (i j : ι'), IsIntegral ℤ (b'.toMatrix (⇑(b.reindex (b.indexEquiv b'))) i j)
hint : IsIntegral ℤ ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
r : ℤ
hr : (algebraMap ℤ ℚ) r = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det
⊢ 1 = ((b.reindex (b.indexEquiv b')).toMatrix ⇑b').det ^ 2