-- Mathlib/NumberTheory/FLT/MasonStothers.lean
theorem Polynomial.abc
    {a b c : k[X]} (ha : a ≠ 0) (hb : b ≠ 0) (hc : c ≠ 0)
    (hab : IsCoprime a b) (hsum : a + b + c = 0) :
    ( natDegree a + 1 ≤ (radical (a * b * c)).natDegree ∧
      natDegree b + 1 ≤ (radical (a * b * c)).natDegree ∧
      natDegree c + 1 ≤ (radical (a * b * c)).natDegree ) ∨
      derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Proof state:
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
⊢ w = c.wronskian a

Tactic:
rw [add_rotate] at hsum

Result:
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : b + c + a = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
⊢ w = c.wronskian a

Proof state:
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
⊢ a.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧
      b.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧ c.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∨
    derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Tactic:
have hbc : IsCoprime b c := by
  rw [add_eq_zero_iff_neg_eq] at hsum
  rw [← hsum, IsCoprime.neg_right_iff]
  convert IsCoprime.add_mul_left_right hab.symm 1
  rw [mul_one]

Result:
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
⊢ a.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧
      b.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧ c.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∨
    derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Proof state:
case h.e'_4.h.e'_6
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : -(b + c) = a
⊢ c = c * 1

Tactic:
rw [mul_one]

Result:
no goals

Proof state:
case neg.h.refine_2
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (a * b * c) ∣ w
hw : ¬w = 0
⊢ b.natDegree + 1 ≤ (radical (a * b * c)).natDegree

Tactic:
rw [← mul_rotate] at abc_dr_dvd_w ⊢

Result:
case neg.h.refine_2
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (c * a * b) ∣ w
hw : ¬w = 0
⊢ b.natDegree + 1 ≤ (radical (c * a * b)).natDegree

Proof state:
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
⊢ a.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧
      b.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧ c.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∨
    derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Tactic:
have hca : IsCoprime c a := by
  rw [add_eq_zero_iff_neg_eq] at hsum'
  rw [← hsum', IsCoprime.neg_right_iff]
  convert IsCoprime.add_mul_left_right hbc.symm 1
  rw [mul_one]

Result:
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
⊢ a.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧
      b.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧ c.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∨
    derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Proof state:
case neg.h.refine_2
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (c * a * b) ∣ w
hw : ¬w = 0
⊢ b.natDegree + 1 ≤ (radical (c * a * b)).natDegree

Tactic:
apply abc_subcall wca <;> assumption

Result:
no goals

Proof state:
case pos
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (a * b * c) ∣ w
hw : w = 0
⊢ a.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧
      b.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∧ c.natDegree + 1 ≤ (radical (a * b * c)).natDegree ∨
    derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Tactic:
right

Result:
case pos.h
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (a * b * c) ∣ w
hw : w = 0
⊢ derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Proof state:
case neg.h.refine_1
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (a * b * c) ∣ w
hw : ¬w = 0
⊢ a.natDegree + 1 ≤ (radical (a * b * c)).natDegree

Tactic:
rw [mul_rotate] at abc_dr_dvd_w ⊢

Result:
case neg.h.refine_1
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : w = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : w = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (b * c * a) ∣ w
hw : ¬w = 0
⊢ a.natDegree + 1 ≤ (radical (b * c * a)).natDegree

Proof state:
case pos.h
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : 0 = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : 0 = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (a * b * c) ∣ w
hw : w = 0
⊢ derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0

Tactic:
obtain ⟨ga, gb⟩ := hab.wronskian_eq_zero_iff.mp wab.symm

Result:
case pos.h.intro
k : Type u_1
inst✝¹ : Field k
inst✝ : DecidableEq k
a b c : k[X]
ha : a ≠ 0
hb : b ≠ 0
hc : c ≠ 0
hab : IsCoprime a b
hsum : a + b + c = 0
w : k[X] := a.wronskian b
wab : 0 = a.wronskian b
hbc : IsCoprime b c
hsum' : b + c + a = 0
hca : IsCoprime c a
wbc : 0 = b.wronskian c
wca : w = c.wronskian a
abc_dr_dvd_w : divRadical (a * b * c) ∣ w
hw : w = 0
ga : derivative a = 0
gb : derivative b = 0
⊢ derivative a = 0 ∧ derivative b = 0 ∧ derivative c = 0