-- Mathlib/Algebra/Polynomial/AlgebraMap.lean
theorem eq_zero_of_mul_eq_zero_of_smul (P : R[X]) (h : ∀ r : R, r • P = 0 → r = 0) (Q : R[X])
    (hQ : P * Q = 0) : Q = 0

Proof state:
case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
⊢ P.coeff l * Q.leadingCoeff = 0

Tactic:
suffices (P * Q).coeff (l + m) = P.coeff l * Q.leadingCoeff by rw [← this, hQ, coeff_zero]

Result:
case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
⊢ (P * Q).coeff (l + m) = P.coeff l * Q.leadingCoeff

Proof state:
case h
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
i : ℕ
hi : i > P.natDegree
⊢ P.coeff i • Q = 0

Tactic:
rw [coeff_eq_zero_of_natDegree_lt hi, zero_smul]

Result:
no goals

Proof state:
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
i j : ℕ
hij : i + j = l + m
H : i = l → ¬j = m
hi : i < l
⊢ m < j

Tactic:
omega

Result:
no goals

Proof state:
case h
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
⊢ ∀ m > P.natDegree, P.coeff m • Q = 0

Tactic:
intro i hi

Result:
case h
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
i : ℕ
hi : i > P.natDegree
⊢ P.coeff i • Q = 0

Proof state:
case step
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
⊢ P.coeff l • Q = 0

Tactic:
obtain _|hl := (natDegree_smul_le (P.coeff l) Q).lt_or_eq

Result:
case step.inl
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
h✝ : (P.coeff l • Q).natDegree < Q.natDegree
⊢ P.coeff l • Q = 0

case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
⊢ P.coeff l • Q = 0

Proof state:
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
this : P.coeff l * Q.leadingCoeff = 0
⊢ P.coeff l • Q = 0

Tactic:
rwa [← leadingCoeff_eq_zero, ← coeff_natDegree, coeff_smul, hl, coeff_natDegree, smul_eq_mul]

Result:
no goals

Proof state:
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
this : ∀ (i : ℕ), P.coeff i • Q = 0
⊢ Q.leadingCoeff = 0

Tactic:
apply h

Result:
case a
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
this : ∀ (i : ℕ), P.coeff i • Q = 0
⊢ Q.leadingCoeff • P = 0

Proof state:
case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
⊢ (P * Q).coeff (l + m) = P.coeff l * Q.leadingCoeff

Tactic:
rw [coeff_mul]

Result:
case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
⊢ ∑ x ∈ antidiagonal (l + m), P.coeff x.1 * Q.coeff x.2 = P.coeff l * Q.leadingCoeff

Proof state:
case inl
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
i j : ℕ
hij : i + j = l + m
H : i = l → ¬j = m
hi : i < l
hj : m < j
⊢ P.coeff i * Q.coeff j = 0

Tactic:
rw [coeff_eq_zero_of_natDegree_lt hj, mul_zero]

Result:
no goals

Proof state:
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
⊢ ∀ (i : ℕ), P.coeff i • Q = 0

Tactic:
apply Nat.strong_decreasing_induction

Result:
case base
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
⊢ ∃ n, ∀ m > n, P.coeff m • Q = 0

case step
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
⊢ ∀ (n : ℕ), (∀ m > n, P.coeff m • Q = 0) → P.coeff n • Q = 0

Proof state:
case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
⊢ P.coeff l * Q.leadingCoeff = 0

Tactic:
let m := Q.natDegree

Result:
case step.inr
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
⊢ P.coeff l * Q.leadingCoeff = 0

Proof state:
case step.inl
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
h✝ : (P.coeff l • Q).natDegree < Q.natDegree
⊢ P.coeff l • Q = 0

Tactic:
apply eq_zero_of_mul_eq_zero_of_smul _ h (P.coeff l • Q)

Result:
case step.inl
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
h✝ : (P.coeff l • Q).natDegree < Q.natDegree
⊢ P * P.coeff l • Q = 0

Proof state:
case step
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
⊢ ∀ (n : ℕ), (∀ m > n, P.coeff m • Q = 0) → P.coeff n • Q = 0

Tactic:
intro l IH

Result:
case step
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
⊢ P.coeff l • Q = 0

Proof state:
case step.inl
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
h✝ : (P.coeff l • Q).natDegree < Q.natDegree
⊢ P * P.coeff l • Q = 0

Tactic:
rw [smul_eq_C_mul, mul_left_comm, hQ, mul_zero]

Result:
no goals

Proof state:
R : Type u
inst✝ : CommSemiring R
P : R[X]
h : ∀ (r : R), r • P = 0 → r = 0
Q : R[X]
hQ : P * Q = 0
l : ℕ
IH : ∀ m > l, P.coeff m • Q = 0
hl : (P.coeff l • Q).natDegree = Q.natDegree
m : ℕ := Q.natDegree
⊢ (l, m) ∉ antidiagonal (l + m) → P.coeff (l, m).1 * Q.coeff (l, m).2 = 0

Tactic:
simp

Result:
no goals