Theorem: Polynomial.content_eq_gcd_leadingCoeff_content_eraseLead
File: Mathlib/RingTheory/Polynomial/Content.lean

tactic: by_cases h : p = 0
state_before: R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content
state_after: case pos
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p = 0
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content

case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : ¬p = 0
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content

tactic: rw [← leadingCoeff_eq_zero, leadingCoeff, ← Ne, ← mem_support_iff] at h
state_before: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : ¬p = 0
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content
state_after: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content

tactic: rw [content, ← Finset.insert_erase h, Finset.gcd_insert, leadingCoeff, content,
  eraseLead_support]
state_before: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content
state_after: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
⊢ gcd (p.coeff p.natDegree) ((p.support.erase p.natDegree).gcd p.coeff) =
    gcd (p.coeff p.natDegree) ((p.support.erase p.natDegree).gcd p.eraseLead.coeff)

tactic: refine congr rfl (Finset.gcd_congr rfl fun i hi => ?_)
state_before: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
⊢ gcd (p.coeff p.natDegree) ((p.support.erase p.natDegree).gcd p.coeff) =
    gcd (p.coeff p.natDegree) ((p.support.erase p.natDegree).gcd p.eraseLead.coeff)
state_after: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
i : ℕ
hi : i ∈ p.support.erase p.natDegree
⊢ p.coeff i = p.eraseLead.coeff i

tactic: rw [Finset.mem_erase] at hi
state_before: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
i : ℕ
hi : i ∈ p.support.erase p.natDegree
⊢ p.coeff i = p.eraseLead.coeff i
state_after: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
i : ℕ
hi : i ≠ p.natDegree ∧ i ∈ p.support
⊢ p.coeff i = p.eraseLead.coeff i

tactic: rw [eraseLead_coeff, if_neg hi.1]
state_before: case neg
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p.natDegree ∈ p.support
i : ℕ
hi : i ≠ p.natDegree ∧ i ∈ p.support
⊢ p.coeff i = p.eraseLead.coeff i
state_after: no goals

tactic: simp [h]
state_before: case pos
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : NormalizedGCDMonoid R
p : R[X]
h : p = 0
⊢ p.content = gcd p.leadingCoeff p.eraseLead.content
state_after: no goals