Theorem: Polynomial.natDegree_cancelLeads_lt_of_natDegree_le_natDegree_of_comm
File: Mathlib/Algebra/Polynomial/CancelLeads.lean

Proof steps:
state_before: R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
⊢ (p.cancelLeads q).natDegree < q.natDegree
state_after: case pos
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : p = 0
⊢ (p.cancelLeads q).natDegree < q.natDegree

case neg
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
⊢ (p.cancelLeads q).natDegree < q.natDegree
tactic: by_cases hp : p = 0
state_before: case neg
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
⊢ (p.cancelLeads q).natDegree < q.natDegree
state_after: case neg
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree < q.natDegree
tactic: rw [cancelLeads, sub_eq_add_neg, tsub_eq_zero_iff_le.mpr h, pow_zero, mul_one]
state_before: case neg
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree < q.natDegree
state_after: case pos
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
h0 : C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p) = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree < q.natDegree

case neg
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
h0 : ¬C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p) = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree < q.natDegree
tactic: by_cases h0 :
    C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p) = 0
state_before: case neg
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
h0 : ¬C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p) = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree < q.natDegree
state_after: case neg.a
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
h0 : ¬C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p) = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree ≤ q.natDegree

case neg.a
R : Type u_1
inst✝ : Ring R
p q : R[X]
comm : p.leadingCoeff * q.leadingCoeff = q.leadingCoeff * p.leadingCoeff
h : p.natDegree ≤ q.natDegree
hq : 0 < q.natDegree
hp : ¬p = 0
h0 : ¬C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p) = 0
⊢ (C p.leadingCoeff * q + -(C q.leadingCoeff * X ^ (q.natDegree - p.natDegree) * p)).natDegree ≠ q.natDegree
tactic: apply lt_of_le_of_ne