-- Mathlib/Algebra/Polynomial/CoeffMem.lean
lemma coeff_divModByMonicAux_mem_span_pow_mul_span : ∀ (p q : S[X]) (hq : q.Monic) (i),
    (p.divModByMonicAux hq).1.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p) ∧
    (p.divModByMonicAux hq).2.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
  | p, q, hq, i => by
    rw [divModByMonicAux]
    have H₀ (i) : p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Proof state:
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
⊢ spanCoeffs(q) ^ deg(r) * (spanCoeffs(q) * spanCoeffs(p)) = spanCoeffs(q) ^ (deg(r) + 1) * spanCoeffs(p)

Tactic:
rw [pow_succ, mul_assoc]

Result:
no goals

Proof state:
case neg.refine_1
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
⊢ (p.leadingCoeff * if i = deg(p) - deg(q) then 1 else 0) ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Tactic:
split_ifs <;> simp only [mul_one, mul_zero]

Result:
case pos
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
h✝ : i = deg(p) - deg(q)
⊢ p.leadingCoeff ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

case neg
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
h✝ : ¬i = deg(p) - deg(q)
⊢ 0 ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Proof state:
case neg
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
⊢ (p.leadingCoeff * if i = deg(p) - deg(q) then 1 else 0) + (r.divModByMonicAux hq).1.coeff i ∈
      spanCoeffs(q) ^ deg(p) * spanCoeffs(p) ∧
    (r.divModByMonicAux hq).2.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Tactic:
refine ⟨add_mem ?_ ?_, ?_⟩

Result:
case neg.refine_1
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
⊢ (p.leadingCoeff * if i = deg(p) - deg(q) then 1 else 0) ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

case neg.refine_2
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
⊢ (r.divModByMonicAux hq).1.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

case neg.refine_3
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
H : span R coeffs(r) ≤ span R coeffs(p) ⊔ span R coeffs(q) * span R coeffs(p)
deg_r_lt_deg_p : deg(r) < deg(p)
H'' : spanCoeffs(q) ^ deg(r) * spanCoeffs(r) ≤ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
⊢ (r.divModByMonicAux hq).2.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Proof state:
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i✝ i : ℕ
⊢ p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Tactic:
refine Submodule.mul_le_mul_left (pow_le_pow_left' le_sup_left _) ?_

Result:
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i✝ i : ℕ
⊢ p.coeff i ∈ 1 ^ deg(p) * spanCoeffs(p)

Proof state:
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i✝ i : ℕ
⊢ p.coeff i ∈ spanCoeffs(p)

Tactic:
exact SetLike.le_def.mp le_sup_right (subset_span (mem_range_self i))

Result:
no goals

Proof state:
case pos
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
⊢ (p.leadingCoeff * if i = deg(p) - deg(q) then 1 else 0) + (r.divModByMonicAux hq).1.coeff i ∈
      spanCoeffs(q) ^ deg(p) * spanCoeffs(p) ∧
    (r.divModByMonicAux hq).2.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

Tactic:
by_cases hr' : r = 0

Result:
case pos
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : r = 0
⊢ (p.leadingCoeff * if i = deg(p) - deg(q) then 1 else 0) + (r.divModByMonicAux hq).1.coeff i ∈
      spanCoeffs(q) ^ deg(p) * spanCoeffs(p) ∧
    (r.divModByMonicAux hq).2.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)

case neg
R : Type u_2
S : Type u_3
inst✝² : CommRing R
inst✝¹ : Ring S
inst✝ : Algebra R S
p q : S[X]
hq : q.Monic
i : ℕ
H₀ : ∀ (i : ℕ), p.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)
hpq : q.degree ≤ p.degree ∧ p ≠ 0
r : S[X]
hr : p - q * (C p.leadingCoeff * X ^ (deg(p) - deg(q))) = r
hr' : ¬r = 0
⊢ (p.leadingCoeff * if i = deg(p) - deg(q) then 1 else 0) + (r.divModByMonicAux hq).1.coeff i ∈
      spanCoeffs(q) ^ deg(p) * spanCoeffs(p) ∧
    (r.divModByMonicAux hq).2.coeff i ∈ spanCoeffs(q) ^ deg(p) * spanCoeffs(p)