-- Mathlib/LinearAlgebra/Matrix/SemiringInverse.lean
theorem detp_mul :
    detp 1 (A * B) + (detp 1 A * detp (-1) B + detp (-1) A * detp 1 B) =
      detp (-1) (A * B) + (detp 1 A * detp 1 B + detp (-1) A * detp (-1) B)

Proof state:
case intro.intro.intro
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hij : i ≠ j
hσ : ∀ (k : n), σ ((swap i j) k) = σ k
⊢ ∑ τ ∈ ofSign 1, ∏ i : n, A i (σ i) * B (σ i) (τ i) =
    ∑ x ∈ ofSign 1, ∏ i_1 : n, A i_1 (σ i_1) * B (σ i_1) (((mulRightEmbedding (swap i j)) x) i_1)

Tactic:
simp_rw [prod_mul_distrib, mulRightEmbedding_apply, Perm.mul_apply]

Result:
case intro.intro.intro
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hij : i ≠ j
hσ : ∀ (k : n), σ ((swap i j) k) = σ k
⊢ ∑ x ∈ ofSign 1, (∏ x : n, A x (σ x)) * ∏ x_1 : n, B (σ x_1) (x x_1) =
    ∑ x ∈ ofSign 1, (∏ x : n, A x (σ x)) * ∏ x_1 : n, B (σ x_1) (x ((swap i j) x_1))

Proof state:
case intro.intro.intro
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hij : i ≠ j
hσ : ∀ (k : n), σ ((swap i j) k) = σ k
τ : Perm n
hτ : τ ∈ ofSign 1
⊢ ∏ i_1 : n, B (σ ((swap i j) i_1)) (τ ((swap i j) i_1)) = ∏ x : n, B (σ x) (τ ((swap i j) x))

Tactic:
simp only [hσ]

Result:
no goals

Proof state:
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
s t : ℤˣ
σ : Perm n
hσ : σ ∈ ofSign s
⊢ ∑ j ∈ ofSign t, (∏ k : n, A k (σ k)) * ∏ k : n, B k (j k) =
    ∑ x ∈ ofSign (t * s), (∏ x : n, A x (σ x)) * ∏ x_1 : n, B (σ x_1) (x x_1)

Tactic:
simp_rw [hf hσ, sum_map, mulRightEmbedding_apply, Perm.mul_apply]

Result:
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
s t : ℤˣ
σ : Perm n
hσ : σ ∈ ofSign s
⊢ ∑ j ∈ ofSign t, (∏ k : n, A k (σ k)) * ∏ k : n, B k (j k) =
    ∑ x ∈ ofSign t, (∏ k : n, A k (σ k)) * ∏ x_1 : n, B (σ x_1) (x (σ x_1))

Proof state:
case intro.intro.intro
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hij : i ≠ j
hσ : ∀ (k : n), σ ((swap i j) k) = σ k
⊢ ∑ τ ∈ ofSign 1, ∏ i : n, A i (σ i) * B (σ i) (τ i) = ∑ τ ∈ ofSign (-1), ∏ i : n, A i (σ i) * B (σ i) (τ i)

Tactic:
rw [← mul_neg_one, hf (mem_ofSign.mpr (sign_swap hij)), sum_map]

Result:
case intro.intro.intro
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hij : i ≠ j
hσ : ∀ (k : n), σ ((swap i j) k) = σ k
⊢ ∑ τ ∈ ofSign 1, ∏ i : n, A i (σ i) * B (σ i) (τ i) =
    ∑ x ∈ ofSign 1, ∏ i_1 : n, A i_1 (σ i_1) * B (σ i_1) (((mulRightEmbedding (swap i j)) x) i_1)

Proof state:
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hσ : σ i = σ j
hij : i ≠ j
k : n
⊢ σ ((swap i j) k) = σ k

Tactic:
rw [swap_apply_def]

Result:
n : Type u_1
R : Type u_3
inst✝² : Fintype n
inst✝¹ : DecidableEq n
inst✝ : CommSemiring R
A B : Matrix n n R
hf : ∀ {s t : ℤˣ} {σ : Perm n}, σ ∈ ofSign s → ofSign (t * s) = Finset.map (mulRightEmbedding σ) (ofSign t)
ι : Perm n ↪ n → n := { toFun := fun e => ⇑e, inj' := ⋯ }
hι : ∀ {σ : Perm n} {x : n}, ι σ x = σ x
bij : Finset (n → n) := Finset.map ι ((ofSign 1).disjUnion (ofSign (-1)) ⋯)
h :
  ∀ (s : ℤˣ),
    detp s (A * B) =
      ∑ σ ∈ bijᶜ, ∑ τ ∈ ofSign s, ∏ i : n, A i (σ i) * B (σ i) (τ i) + (detp 1 A * detp s B + detp (-1) A * detp (-s) B)
σ : n → n
hσ✝ : ¬Function.Injective σ
i j : n
hσ : σ i = σ j
hij : i ≠ j
k : n
⊢ σ (if k = i then j else if k = j then i else k) = σ k