-- Mathlib/LinearAlgebra/Matrix/Transvection.lean
theorem diagonal_transvection_induction (P : Matrix n n 𝕜 → Prop) (M : Matrix n n 𝕜)
    (hdiag : ∀ D : n → 𝕜, det (diagonal D) = det M → P (diagonal D))
    (htransvec : ∀ t : TransvectionStruct n 𝕜, P t.toMatrix) (hmul : ∀ A B, P A → P B → P (A * B)) :
    P M

Proof state:
case intro.intro.intro.cons
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
L₂ : List (TransvectionStruct n 𝕜)
E : Matrix n n 𝕜
PE : P E
t : TransvectionStruct n 𝕜
L₁ : List (TransvectionStruct n 𝕜)
IH : P ((List.map toMatrix L₁).prod * E * (List.map toMatrix L₂).prod)
⊢ P ((List.map toMatrix (t :: L₁)).prod * E * (List.map toMatrix L₂).prod)

Tactic:
simp only [Matrix.mul_assoc, List.prod_cons, List.map] at IH ⊢

Result:
case intro.intro.intro.cons
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
L₂ : List (TransvectionStruct n 𝕜)
E : Matrix n n 𝕜
PE : P E
t : TransvectionStruct n 𝕜
L₁ : List (TransvectionStruct n 𝕜)
IH : P ((List.map toMatrix L₁).prod * (E * (List.map toMatrix L₂).prod))
⊢ P (t.toMatrix * ((List.map toMatrix L₁).prod * (E * (List.map toMatrix L₂).prod)))

Proof state:
case intro.intro.intro.nil
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
L₂ : List (TransvectionStruct n 𝕜)
E : Matrix n n 𝕜
PE : P E
⊢ P ((List.map toMatrix []).prod * E * (List.map toMatrix L₂).prod)

Tactic:
simp only [Matrix.one_mul, List.prod_nil, List.map]

Result:
case intro.intro.intro.nil
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
L₂ : List (TransvectionStruct n 𝕜)
E : Matrix n n 𝕜
PE : P E
⊢ P (E * (List.map toMatrix L₂).prod)

Proof state:
case intro.intro.intro.nil.nil
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
E : Matrix n n 𝕜
PE : P E
⊢ P (E * (List.map toMatrix []).prod)

Tactic:
simpa

Result:
no goals

Proof state:
case intro.intro.intro.nil.cons
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
t : TransvectionStruct n 𝕜
L₂ : List (TransvectionStruct n 𝕜)
IH : ∀ (E : Matrix n n 𝕜), P E → P (E * (List.map toMatrix L₂).prod)
E : Matrix n n 𝕜
PE : P E
⊢ P (E * t.toMatrix * (List.map toMatrix L₂).prod)

Tactic:
apply IH

Result:
case intro.intro.intro.nil.cons.PE
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
t : TransvectionStruct n 𝕜
L₂ : List (TransvectionStruct n 𝕜)
IH : ∀ (E : Matrix n n 𝕜), P E → P (E * (List.map toMatrix L₂).prod)
E : Matrix n n 𝕜
PE : P E
⊢ P (E * t.toMatrix)

Proof state:
case intro.intro.intro
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
⊢ P M

Tactic:
suffices H :
  ∀ (L₁ L₂ : List (TransvectionStruct n 𝕜)) (E : Matrix n n 𝕜),
    P E → P ((L₁.map toMatrix).prod * E * (L₂.map toMatrix).prod) by
  rw [h]
  apply H L L'
  exact PD

Result:
case intro.intro.intro
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
⊢ ∀ (L₁ L₂ : List (TransvectionStruct n 𝕜)) (E : Matrix n n 𝕜),
    P E → P ((List.map toMatrix L₁).prod * E * (List.map toMatrix L₂).prod)

Proof state:
case intro.intro.intro
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
⊢ ∀ (L₁ L₂ : List (TransvectionStruct n 𝕜)) (E : Matrix n n 𝕜),
    P E → P ((List.map toMatrix L₁).prod * E * (List.map toMatrix L₂).prod)

Tactic:
intro L₁ L₂ E PE

Result:
case intro.intro.intro
n : Type u_1
𝕜 : Type u_3
inst✝² : Field 𝕜
inst✝¹ : DecidableEq n
inst✝ : Fintype n
P : Matrix n n 𝕜 → Prop
M : Matrix n n 𝕜
hdiag : ∀ (D : n → 𝕜), (diagonal D).det = M.det → P (diagonal D)
htransvec : ∀ (t : TransvectionStruct n 𝕜), P t.toMatrix
hmul : ∀ (A B : Matrix n n 𝕜), P A → P B → P (A * B)
L L' : List (TransvectionStruct n 𝕜)
D : n → 𝕜
h : M = (List.map toMatrix L).prod * diagonal D * (List.map toMatrix L').prod
PD : P (diagonal D)
L₁ L₂ : List (TransvectionStruct n 𝕜)
E : Matrix n n 𝕜
PE : P E
⊢ P ((List.map toMatrix L₁).prod * E * (List.map toMatrix L₂).prod)