-- Mathlib/RingTheory/Discriminant.lean
theorem discr_powerBasis_eq_norm [Algebra.IsSeparable K L] :
    discr K pb.basis =
      (-1) ^ (n * (n - 1) / 2) *
      norm K (aeval pb.gen (derivative (R := K) (minpoly K pb.gen)))

Proof state:
case a.e_a
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots : ∀ (σ : L →ₐ[K] E), σ pb.gen ∈ (minpoly K pb.gen).aroots E
⊢ ∏ x ∈ univ.sigma fun i => {i}ᶜ, ((e x.snd) pb.gen - (e x.fst) pb.gen) =
    ∏ x ∈ univ.sigma fun σ => { val := ((minpoly K pb.gen).aroots E).erase (σ pb.gen), nodup := ⋯ },
      (x.fst pb.gen - x.snd)

Tactic:
refine prod_bij' (fun i _ ↦ ⟨e i.2, e i.1 pb.gen⟩)
  (fun σ hσ ↦ ⟨e.symm (PowerBasis.lift pb σ.2 ?_), e.symm σ.1⟩) ?_ ?_ ?_ ?_ (fun i _ ↦ by simp)
<;> simp only [mem_sigma, mem_univ, Finset.mem_mk, hnodup.mem_erase_iff, IsRoot.def,
    mem_roots', minpoly.ne_zero (Algebra.IsSeparable.isIntegral K pb.gen), not_false_eq_true,
    mem_singleton, true_and, @mem_compl _ _ _ (_), Sigma.forall, Equiv.apply_symm_apply,
    PowerBasis.lift_gen, and_imp, implies_true, forall_const, Equiv.symm_apply_apply,
    Sigma.ext_iff, Equiv.symm_apply_eq, heq_eq_eq, and_true] at *

Result:
case a.e_a.refine_1
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
σ : (_ : L →ₐ[K] E) × E
hroots :
  ∀ (σ : L →ₐ[K] E),
    Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0 ∧
      eval (σ pb.gen) (Polynomial.map (algebraMap K E) (minpoly K pb.gen)) = 0
hσ :
  σ.snd ≠ σ.fst pb.gen ∧
    Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0 ∧
      eval σ.snd (Polynomial.map (algebraMap K E) (minpoly K pb.gen)) = 0
⊢ (aeval σ.snd) (minpoly K pb.gen) = 0

case a.e_a.refine_2
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots :
  ∀ (σ : L →ₐ[K] E),
    Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0 ∧
      eval (σ pb.gen) (Polynomial.map (algebraMap K E) (minpoly K pb.gen)) = 0
⊢ ∀ (a b : Fin pb.dim),
    ¬b = a →
      (e a) pb.gen ≠ (e b) pb.gen ∧
        Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0 ∧
          eval ((e a) pb.gen) (Polynomial.map (algebraMap K E) (minpoly K pb.gen)) = 0

case a.e_a.refine_3
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots✝ : ∀ (σ : L →ₐ[K] E), σ pb.gen ∈ (minpoly K pb.gen).aroots E
hroots :
  ∀ (σ : L →ₐ[K] E),
    Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0 ∧
      eval (σ pb.gen) (Polynomial.map (algebraMap K E) (minpoly K pb.gen)) = 0
⊢ ∀ (a : L →ₐ[K] E) (b : E)
    (ha :
      b ≠ a pb.gen ∧
        Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0 ∧
          eval b (Polynomial.map (algebraMap K E) (minpoly K pb.gen)) = 0),
    ¬a = pb.lift b ⋯

case a.e_a.refine_4
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots : ∀ (σ : L →ₐ[K] E), σ pb.gen ∈ (minpoly K pb.gen).aroots E
⊢ ∀ (a b : Fin pb.dim) (ha : ¬b = a), pb.lift ((e a) pb.gen) ⋯ = e a

Proof state:
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots : ∀ (σ : L →ₐ[K] E), σ pb.gen ∈ (minpoly K pb.gen).aroots E
⊢ discr K ⇑pb.basis = (-1) ^ (n * (n - 1) / 2) * (norm K) ((aeval pb.gen) (derivative (minpoly K pb.gen)))

Tactic:
apply (algebraMap K E).injective

Result:
case a
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots : ∀ (σ : L →ₐ[K] E), σ pb.gen ∈ (minpoly K pb.gen).aroots E
⊢ (algebraMap K E) (discr K ⇑pb.basis) =
    (algebraMap K E) ((-1) ^ (n * (n - 1) / 2) * (norm K) ((aeval pb.gen) (derivative (minpoly K pb.gen))))

Proof state:
case a.e_a.refine_4
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
hroots : ∀ (σ : L →ₐ[K] E), σ pb.gen ∈ (minpoly K pb.gen).aroots E
⊢ ∀ (a b : Fin pb.dim) (ha : ¬b = a), pb.lift ((e a) pb.gen) ⋯ = e a

Tactic:
exact fun a b _ ↦ pb.algHom_ext <| pb.lift_gen _ _

Result:
no goals

Proof state:
K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
σ : L →ₐ[K] E
⊢ σ ((aeval pb.gen) (minpoly K pb.gen)) = 0

K : Type u
L : Type v
inst✝⁴ : Field K
inst✝³ : Field L
inst✝² : Algebra K L
inst✝¹ : Module.Finite K L
pb : PowerBasis K L
inst✝ : Algebra.IsSeparable K L
E : Type v := AlgebraicClosure L
this : (a b : E) → Decidable (a = b) := fun a b => Classical.propDecidable (a = b)
e : Fin pb.dim ≃ (L →ₐ[K] E)
hnodup : ((minpoly K pb.gen).aroots E).Nodup
σ : L →ₐ[K] E
⊢ Polynomial.map (algebraMap K E) (minpoly K pb.gen) ≠ 0

Tactic:
repeat' simp [minpoly.ne_zero (Algebra.IsSeparable.isIntegral K pb.gen)]

Result:
no goals