-- Mathlib/RingTheory/Norm/Basic.lean
theorem norm_eq_zero_iff [IsDomain R] [IsDomain S] [Module.Free R S] [Module.Finite R S] {x : S} :
    norm R x = 0 ↔ x = 0

Proof state:
case mp.intro.intro.refine_2.a
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
decEq : DecidableEq (Free.ChooseBasisIndex R S) := Classical.decEq (Free.ChooseBasisIndex R S)
v : Free.ChooseBasisIndex R S → R
hv : ⇑(b.repr (x * ∑ i : Free.ChooseBasisIndex R S, v i • b i)) = 0
sum_eq : ∑ i : Free.ChooseBasisIndex R S, v i • b i = 0
⊢ b.equivFun.symm v = b.equivFun.symm 0

Tactic:
rw [b.equivFun_symm_apply, sum_eq, LinearEquiv.map_zero]

Result:
no goals

Proof state:
case mp.intro.intro.refine_1
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
decEq : DecidableEq (Free.ChooseBasisIndex R S) := Classical.decEq (Free.ChooseBasisIndex R S)
v : Free.ChooseBasisIndex R S → R
v_ne : v ≠ 0
hv : ⇑(b.repr (x * ∑ i : Free.ChooseBasisIndex R S, v i • b i)) = 0
i : Free.ChooseBasisIndex R S
⊢ (b.repr (x * ∑ i : Free.ChooseBasisIndex R S, v i • b i)) i = (b.repr 0) i

Tactic:
simpa only [LinearEquiv.map_zero, Pi.zero_apply] using congr_fun hv i

Result:
no goals

Proof state:
case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
⊢ (norm R) x = 0 → x = 0

Tactic:
let decEq := Classical.decEq (Module.Free.ChooseBasisIndex R S)

Result:
case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
decEq : DecidableEq (Free.ChooseBasisIndex R S) := Classical.decEq (Free.ChooseBasisIndex R S)
⊢ (norm R) x = 0 → x = 0

Proof state:
case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
⊢ (norm R) x = 0 → x = 0

case mpr
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
⊢ x = 0 → (norm R) x = 0

Tactic:
swap

Result:
case mpr
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
⊢ x = 0 → (norm R) x = 0

case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
⊢ (norm R) x = 0 → x = 0

Proof state:
case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
⊢ (norm R) x = 0 → x = 0

Tactic:
let b := Module.Free.chooseBasis R S

Result:
case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
⊢ (norm R) x = 0 → x = 0

Proof state:
case mp
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
decEq : DecidableEq (Free.ChooseBasisIndex R S) := Classical.decEq (Free.ChooseBasisIndex R S)
⊢ (∃ v, v ≠ 0 ∧ (leftMulMatrix b) x *ᵥ v = 0) → x = 0

Tactic:
rintro ⟨v, v_ne, hv⟩

Result:
case mp.intro.intro
R : Type u_1
S : Type u_2
inst✝⁶ : CommRing R
inst✝⁵ : Ring S
inst✝⁴ : Algebra R S
inst✝³ : IsDomain R
inst✝² : IsDomain S
inst✝¹ : Free R S
inst✝ : Module.Finite R S
x : S
b : Basis (Free.ChooseBasisIndex R S) R S := Free.chooseBasis R S
decEq : DecidableEq (Free.ChooseBasisIndex R S) := Classical.decEq (Free.ChooseBasisIndex R S)
v : Free.ChooseBasisIndex R S → R
v_ne : v ≠ 0
hv : (leftMulMatrix b) x *ᵥ v = 0
⊢ x = 0