-- Mathlib/RingTheory/ChainOfDivisors.lean
theorem second_of_chain_is_irreducible {q : Associates M} {n : ℕ} (hn : n ≠ 0)
    {c : Fin (n + 1) → Associates M} (h₁ : StrictMono c) (h₂ : ∀ {r}, r ≤ q ↔ ∃ i, r = c i)
    (hq : q ≠ 0) : Irreducible (c 1)

Proof state:
case succ.refine_2.intro.mk
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
i : ℕ
hi : i < n + 1 + 1
hb : c ⟨i, hi⟩ < c 1
⊢ c ⟨i, hi⟩ = ⊥

Tactic:
cases i

Result:
case succ.refine_2.intro.mk.zero
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
hi : 0 < n + 1 + 1
hb : c ⟨0, hi⟩ < c 1
⊢ c ⟨0, hi⟩ = ⊥

case succ.refine_2.intro.mk.succ
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
n✝ : ℕ
hi : n✝ + 1 < n + 1 + 1
hb : c ⟨n✝ + 1, hi⟩ < c 1
⊢ c ⟨n✝ + 1, hi⟩ = ⊥

Proof state:
case succ.refine_2.intro.mk.succ
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
n✝ : ℕ
hi : n✝ + 1 < n + 1 + 1
hb : c ⟨n✝ + 1, hi⟩ < c 1
⊢ c ⟨n✝ + 1, hi⟩ = ⊥

Tactic:
simpa [Fin.lt_iff_val_lt_val] using h₁.lt_iff_lt.mp hb

Result:
no goals

Proof state:
case succ.refine_2.intro.mk.zero
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
hi : 0 < n + 1 + 1
hb : c ⟨0, hi⟩ < c 1
⊢ c ⟨0, hi⟩ = ⊥

Tactic:
exact (Associates.isUnit_iff_eq_one _).mp (first_of_chain_isUnit h₁ @h₂)

Result:
no goals

Proof state:
case succ.refine_2
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
b : Associates M
hb : b < c 1
⊢ b = ⊥

Tactic:
obtain ⟨⟨i, hi⟩, rfl⟩ := h₂.1 (hb.le.trans (h₂.2 ⟨1, rfl⟩))

Result:
case succ.refine_2.intro.mk
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
i : ℕ
hi : i < n + 1 + 1
hb : c ⟨i, hi⟩ < c 1
⊢ c ⟨i, hi⟩ = ⊥

Proof state:
case succ.refine_1
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
⊢ c 1 ≠ ⊥

Tactic:
exact ne_bot_of_gt (h₁ (show (0 : Fin (n + 2)) < 1 from Fin.one_pos))

Result:
no goals

Proof state:
case zero
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
hn : 0 ≠ 0
c : Fin (0 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
⊢ Irreducible (c 1)

Tactic:
contradiction

Result:
no goals

Proof state:
case succ
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
⊢ Irreducible (c 1)

Tactic:
refine (Associates.isAtom_iff (ne_zero_of_dvd_ne_zero hq (h₂.2 ⟨1, rfl⟩))).mp ⟨?_, fun b hb => ?_⟩

Result:
case succ.refine_1
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
⊢ c 1 ≠ ⊥

case succ.refine_2
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
b : Associates M
hb : b < c 1
⊢ b = ⊥

Proof state:
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
n : ℕ
hn : n ≠ 0
c : Fin (n + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
hq : q ≠ 0
⊢ Irreducible (c 1)

Tactic:
rcases n with - | n

Result:
case zero
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
hn : 0 ≠ 0
c : Fin (0 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
⊢ Irreducible (c 1)

case succ
M : Type u_1
inst✝ : CancelCommMonoidWithZero M
q : Associates M
hq : q ≠ 0
n : ℕ
hn : n + 1 ≠ 0
c : Fin (n + 1 + 1) → Associates M
h₁ : StrictMono c
h₂ : ∀ {r : Associates M}, r ≤ q ↔ ∃ i, r = c i
⊢ Irreducible (c 1)