-- Mathlib/Algebra/MonoidAlgebra/Degree.lean
lemma leadingCoeff_mul [NoZeroDivisors R]
    (hD : D.Injective) (hadd : ∀ a1 a2, D (a1 + a2) = D a1 + D a2) :
    (p * q).leadingCoeff D = p.leadingCoeff D * q.leadingCoeff D

Proof state:
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
⊢ leadingCoeff D (p * q) = leadingCoeff D p * leadingCoeff D q

Tactic:
obtain rfl | hp := eq_or_ne p 0

Result:
case inl
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
⊢ leadingCoeff D (0 * q) = leadingCoeff D 0 * leadingCoeff D q

case inr
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
⊢ leadingCoeff D (p * q) = leadingCoeff D p * leadingCoeff D q

Proof state:
case inr.inl
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
⊢ leadingCoeff D (p * 0) = leadingCoeff D p * leadingCoeff D 0

Tactic:
simp_rw [leadingCoeff_zero, mul_zero, leadingCoeff_zero]

Result:
no goals

Proof state:
case inr
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
⊢ leadingCoeff D (p * q) = leadingCoeff D p * leadingCoeff D q

Tactic:
obtain rfl | hq := eq_or_ne q 0

Result:
case inr.inl
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
⊢ leadingCoeff D (p * 0) = leadingCoeff D p * leadingCoeff D 0

case inr.inr
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
hq : q ≠ 0
⊢ leadingCoeff D (p * q) = leadingCoeff D p * leadingCoeff D q

Proof state:
case inl
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
⊢ leadingCoeff D (0 * q) = leadingCoeff D 0 * leadingCoeff D q

Tactic:
simp_rw [leadingCoeff_zero, zero_mul, leadingCoeff_zero]

Result:
no goals

Proof state:
case inr.inr
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
hq : q ≠ 0
⊢ leadingCoeff D (p * q) = leadingCoeff D p * leadingCoeff D q

Tactic:
rw [← apply_supDegree_add_supDegree hD hadd, ← supDegree_mul hD hadd ?_ hp hq, leadingCoeff]

Result:
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
hq : q ≠ 0
⊢ leadingCoeff D p * leadingCoeff D q ≠ 0

Proof state:
R : Type u_1
A : Type u_3
B : Type u_5
inst✝⁷ : Semiring R
inst✝⁶ : LinearOrder B
inst✝⁵ : OrderBot B
p q : R[A]
D : A → B
inst✝⁴ : AddZeroClass A
inst✝³ : Add B
inst✝² : AddLeftStrictMono B
inst✝¹ : AddRightStrictMono B
inst✝ : NoZeroDivisors R
hD : Function.Injective D
hadd : ∀ (a1 a2 : A), D (a1 + a2) = D a1 + D a2
hp : p ≠ 0
hq : q ≠ 0
⊢ leadingCoeff D p * leadingCoeff D q ≠ 0

Tactic:
apply mul_ne_zero <;> rwa [Ne, leadingCoeff_eq_zero hD]

Result:
no goals