-- Mathlib/NumberTheory/FLT/Three.lean
private lemma three_dvd_b_of_dvd_a_of_gcd_eq_one_of_case2 {a b c : ℤ} (ha : a ≠ 0)
    (Hgcd : Finset.gcd {a, b, c} id = 1) (h3a : 3 ∣ a) (HF : a ^ 3 + b ^ 3 + c ^ 3 = 0)
    (H : ∀ a b c : ℤ, c ≠ 0 → ¬ 3 ∣ a → ¬ 3 ∣ b  → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3) :
    3 ∣ b

Proof state:
case pos
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : 3 ∣ c
⊢ False

Tactic:
apply h3b

Result:
case pos
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : 3 ∣ c
⊢ 3 ∣ b

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
⊢ IsCoprime b c

Tactic:
rw [add_comm (a ^ 3), add_assoc, add_comm (a ^ 3), ← add_assoc] at HF

Result:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : b ^ 3 + c ^ 3 + a ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
⊢ IsCoprime b c

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
⊢ 3 ∣ b

Tactic:
by_contra! h3b

Result:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
⊢ False

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : b ^ 3 + c ^ 3 + a ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
⊢ IsCoprime b c

Tactic:
refine isCoprime_of_gcd_eq_one_of_FLT ?_ HF

Result:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : b ^ 3 + c ^ 3 + a ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
⊢ {b, c, a}.gcd id = 1

Proof state:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ False

Tactic:
refine H (-b) (-c) a ha (by simp [h3b]) (by simp [h3c]) h3a hbc ?_

Result:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ (-b) ^ 3 + (-c) ^ 3 = a ^ 3

Proof state:
case pos
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : 3 ∣ c
⊢ 3 ∣ b

Tactic:
rw [add_assoc, add_comm (b ^ 3), ← add_assoc] at HF

Result:
case pos
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + c ^ 3 + b ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : 3 ∣ c
⊢ 3 ∣ b

Proof state:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 = (-c) ^ 3
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ (-b) ^ 3 + (-c) ^ 3 = a ^ 3

Tactic:
rw [← HF]

Result:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 = (-c) ^ 3
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ (-b) ^ 3 + (a ^ 3 + b ^ 3) = a ^ 3

Proof state:
case h.e'_2.h.e'_5
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : b ^ 3 + c ^ 3 + a ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
⊢ {b, c, a} = {a, b, c}

Tactic:
rw [Finset.pair_comm, Finset.insert_comm]

Result:
no goals

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
⊢ False

Tactic:
by_cases h3c : 3 ∣ c

Result:
case pos
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : 3 ∣ c
⊢ False

case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ False

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
⊢ 3 ∣ b

Tactic:
have hbc : IsCoprime (-b) (-c) := by
  refine IsCoprime.neg_neg ?_
  rw [add_comm (a ^ 3), add_assoc, add_comm (a ^ 3), ← add_assoc] at HF
  refine isCoprime_of_gcd_eq_one_of_FLT ?_ HF
  convert Hgcd using 2
  rw [Finset.pair_comm, Finset.insert_comm]

Result:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
⊢ 3 ∣ b

Proof state:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ (-b) ^ 3 + (-c) ^ 3 = a ^ 3

Tactic:
rw [add_eq_zero_iff_eq_neg, ← (show Odd 3 by decide).neg_pow] at HF

Result:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 = (-c) ^ 3
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ (-b) ^ 3 + (-c) ^ 3 = a ^ 3

Proof state:
case pos
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + c ^ 3 + b ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : 3 ∣ c
⊢ 3 ∣ b

Tactic:
exact dvd_c_of_prime_of_dvd_a_of_dvd_b_of_FLT Int.prime_three h3a h3c HF

Result:
no goals

Proof state:
case neg
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 = (-c) ^ 3
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ (-b) ^ 3 + (a ^ 3 + b ^ 3) = a ^ 3

Tactic:
ring

Result:
no goals

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 + c ^ 3 = 0
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ ¬3 ∣ -c

Tactic:
simp [h3c]

Result:
no goals

Proof state:
a b c : ℤ
ha : a ≠ 0
Hgcd : {a, b, c}.gcd id = 1
h3a : 3 ∣ a
HF : a ^ 3 + b ^ 3 = -c ^ 3
H : ∀ (a b c : ℤ), c ≠ 0 → ¬3 ∣ a → ¬3 ∣ b → 3 ∣ c → IsCoprime a b → a ^ 3 + b ^ 3 ≠ c ^ 3
hbc : IsCoprime (-b) (-c)
h3b : ¬3 ∣ b
h3c : ¬3 ∣ c
⊢ Odd 3

Tactic:
decide

Result:
no goals