-- Mathlib/LinearAlgebra/Matrix/FixedDetMatrices.lean
theorem induction_on {C : Δ m → Prop} {A : Δ m} (hm : m ≠ 0)
    (h0 : ∀ A : Δ m, A.1 1 0 = 0 → 0 < A.1 0 0 → 0 ≤ A.1 0 1 → |(A.1 0 1)| < |(A.1 1 1)| → C A)
    (hS : ∀ B, C B → C (S • B)) (hT : ∀ B, C B → C (T • B)) : C A

Proof state:
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
⊢ C (reduce A)

Tactic:
rcases reduce_mem_reps hm A with ⟨H1, H2, H3, H4⟩

Result:
case intro.intro.intro
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
H1 : ↑(reduce A) 1 0 = 0
H2 : 0 < ↑(reduce A) 0 0
H3 : 0 ≤ ↑(reduce A) 0 1
H4 : |↑(reduce A) 0 1| < |↑(reduce A) 1 1|
⊢ C (reduce A)

Proof state:
case step
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
hc : ↑A 1 0 ≠ 0
ih : C (reduce (reduceStep A)) → C (reduceStep A)
hA : C (reduce A)
⊢ C A

Tactic:
rw [← reduce_reduceStep hc] at hA

Result:
case step
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
hc : ↑A 1 0 ≠ 0
ih : C (reduce (reduceStep A)) → C (reduceStep A)
hA : C (reduce (reduceStep A))
⊢ C A

Proof state:
case step
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
hc : ↑A 1 0 ≠ 0
ih : C (reduce (reduceStep A)) → C (reduceStep A)
hA : C (reduce (reduceStep A))
⊢ C A

Tactic:
simpa only [reduceStep, prop_red_S hS, prop_red_T_pow hS hT] using ih hA

Result:
no goals

Proof state:
case intro.intro.intro
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
H1 : ↑(reduce A) 1 0 = 0
H2 : 0 < ↑(reduce A) 0 0
H3 : 0 ≤ ↑(reduce A) 0 1
H4 : |↑(reduce A) 0 1| < |↑(reduce A) 1 1|
⊢ C (reduce A)

Tactic:
exact h0 _ H1 H2 H3 H4

Result:
no goals

Proof state:
case pos
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
h : ↑A 1 0 = 0
h1 : 0 < ↑A 0 0
⊢ C (reduce A) → C A

Tactic:
simp only [reduce_of_pos h h1, prop_red_T_pow hS hT, imp_self]

Result:
no goals

Proof state:
case base
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A)
⊢ ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → C (reduce A) → C A

Tactic:
intro A h

Result:
case base
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
h : ↑A 1 0 = 0
⊢ C (reduce A) → C A

Proof state:
case step
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A)
⊢ ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 ≠ 0 → (C (reduce (reduceStep A)) → C (reduceStep A)) → C (reduce A) → C A

Tactic:
intro A hc ih hA

Result:
case step
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
hc : ↑A 1 0 ≠ 0
ih : C (reduce (reduceStep A)) → C (reduceStep A)
hA : C (reduce A)
⊢ C A

Proof state:
case neg
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
h : ↑A 1 0 = 0
h1 : ¬0 < ↑A 0 0
⊢ C (reduce A) → C A

Tactic:
simp only [reduce_of_not_pos h h1, prop_red_T_pow hS hT, prop_red_S hS, imp_self]

Result:
no goals

Proof state:
case base
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
h : ↑A 1 0 = 0
⊢ C (reduce A) → C A

Tactic:
by_cases h1 : 0 < A.1 0 0

Result:
case pos
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
h : ↑A 1 0 = 0
h1 : 0 < ↑A 0 0
⊢ C (reduce A) → C A

case neg
m : ℤ
C : FixedDetMatrix (Fin 2) ℤ m → Prop
A✝ : FixedDetMatrix (Fin 2) ℤ m
hm : m ≠ 0
h0 : ∀ (A : FixedDetMatrix (Fin 2) ℤ m), ↑A 1 0 = 0 → 0 < ↑A 0 0 → 0 ≤ ↑A 0 1 → |↑A 0 1| < |↑A 1 1| → C A
hS : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (S • B)
hT : ∀ (B : FixedDetMatrix (Fin 2) ℤ m), C B → C (T • B)
h_reduce : C (reduce A✝)
A : FixedDetMatrix (Fin 2) ℤ m
h : ↑A 1 0 = 0
h1 : ¬0 < ↑A 0 0
⊢ C (reduce A) → C A