-- Mathlib/LinearAlgebra/FreeModule/Norm.lean
theorem associated_norm_prod_smith [Fintype ι] (b : Basis ι R S) {f : S} (hf : f ≠ 0) :
    Associated (Algebra.norm R f) (∏ i, smithCoeffs b _ (span_singleton_eq_bot.not.2 hf) i)

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
i : ι
⊢ smithCoeffs b (span {f}) ⋯ i • b' i =
    (↑(Algebra.lmul R S).toRingHom f)
      ((LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)) ((selfBasis b (span {f}) hI) ((Equiv.refl ι) i)))

Tactic:
change _ = f * _

Result:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
i : ι
⊢ smithCoeffs b (span {f}) ⋯ i • b' i =
    f * (LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)) ((selfBasis b (span {f}) hI) ((Equiv.refl ι) i))

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
⊢ Associated ((Algebra.norm R) f) (∏ i : ι, smithCoeffs b (span {f}) ⋯ i)

Tactic:
let b' := ringBasis b (span {f}) hI

Result:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
⊢ Associated ((Algebra.norm R) f) (∏ i : ι, smithCoeffs b (span {f}) ⋯ i)

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
i : ι
⊢ smithCoeffs b (span {f}) ⋯ i • b' i =
    smithCoeffs b (span {f}) hI ((Equiv.refl ι) i) • (ringBasis b (span {f}) hI) ((Equiv.refl ι) i)

Tactic:
rfl

Result:
no goals

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
i : ι
⊢ smithCoeffs b (span {f}) ⋯ i • b' i =
    f * (LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)) ((selfBasis b (span {f}) hI) ((Equiv.refl ι) i))

Tactic:
rw [mul_comm, ← smul_eq_mul, LinearEquiv.restrictScalars_apply]

Result:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
i : ι
⊢ smithCoeffs b (span {f}) ⋯ i • b' i =
    (LinearEquiv.coord S S f hf) ((selfBasis b (span {f}) hI) ((Equiv.refl ι) i)) • f

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
⊢ ↑(Matrix.toLin b' b') (Matrix.diagonal (smithCoeffs b (span {f}) ⋯)) =
    ↑(Algebra.lmul R S).toRingHom f ∘ₗ
      ↑(LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)) ∘ₗ
        ↑(b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι))

Tactic:
refine b'.ext fun i => ?_

Result:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
i : ι
⊢ (↑(Matrix.toLin b' b') (Matrix.diagonal (smithCoeffs b (span {f}) ⋯))) (b' i) =
    (↑(Algebra.lmul R S).toRingHom f ∘ₗ
        ↑(LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)) ∘ₗ
          ↑(b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι)))
      (b' i)

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
⊢ Associated ((Algebra.norm R) f) (∏ i : ι, smithCoeffs b (span {f}) ⋯ i)

Tactic:
have hI := span_singleton_eq_bot.not.2 hf

Result:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
⊢ Associated ((Algebra.norm R) f) (∏ i : ι, smithCoeffs b (span {f}) ⋯ i)

Proof state:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
⊢ ∀ (x : S),
    ((Matrix.toLin b' b') (Matrix.diagonal (smithCoeffs b (span {f}) ⋯))) x = (↑(Algebra.lmul R S).toRingHom f) (e x)

Tactic:
dsimp only [e, LinearEquiv.trans_apply]

Result:
R : Type u_1
S : Type u_2
ι : Type u_3
inst✝⁶ : CommRing R
inst✝⁵ : IsDomain R
inst✝⁴ : IsPrincipalIdealRing R
inst✝³ : CommRing S
inst✝² : IsDomain S
inst✝¹ : Algebra R S
inst✝ : Fintype ι
b : Basis ι R S
f : S
hf : f ≠ 0
hI : ¬span {f} = ⊥
b' : Basis ι R S := ringBasis b (span {f}) hI
e : S ≃ₗ[R] S :=
  b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι) ≪≫ₗ LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf)
⊢ ∀ (x : S),
    ((Matrix.toLin b' b') (Matrix.diagonal (smithCoeffs b (span {f}) ⋯))) x =
      (↑(Algebra.lmul R S).toRingHom f)
        ((LinearEquiv.restrictScalars R (LinearEquiv.coord S S f hf))
          ((b'.equiv (selfBasis b (span {f}) hI) (Equiv.refl ι)) x))