Theorem: Polynomial.div_modByMonic_unique
File: Mathlib/Algebra/Polynomial/Div.lean

Proof steps:
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
⊢ f /ₘ g = q ∧ f %ₘ g = r
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
⊢ f /ₘ g = q ∧ f %ₘ g = r
tactic: nontriviality R
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
⊢ f /ₘ g = q ∧ f %ₘ g = r
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
⊢ f /ₘ g = q ∧ f %ₘ g = r
tactic: have h₁ : r - f %ₘ g = -g * (q - f /ₘ g) :=
    eq_of_sub_eq_zero
      (by
        rw [← sub_eq_zero_of_eq (h.1.trans (modByMonic_add_div f hg).symm)]
        simp [mul_add, mul_comm, sub_eq_add_neg, add_comm, add_left_comm, add_assoc])
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
⊢ f /ₘ g = q ∧ f %ₘ g = r
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
h₂ : (r - f %ₘ g).degree = (g * (q - f /ₘ g)).degree
⊢ f /ₘ g = q ∧ f %ₘ g = r
tactic: have h₂ : degree (r - f %ₘ g) = degree (g * (q - f /ₘ g)) := by simp [h₁]
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
h₂ : (r - f %ₘ g).degree = (g * (q - f /ₘ g)).degree
⊢ f /ₘ g = q ∧ f %ₘ g = r
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
h₂ : (r - f %ₘ g).degree = (g * (q - f /ₘ g)).degree
h₄ : (r - f %ₘ g).degree < g.degree
⊢ f /ₘ g = q ∧ f %ₘ g = r
tactic: have h₄ : degree (r - f %ₘ g) < degree g :=
    calc
      degree (r - f %ₘ g) ≤ max (degree r) (degree (f %ₘ g)) := degree_sub_le _ _
      _ < degree g := max_lt_iff.2 ⟨h.2, degree_modByMonic_lt _ hg⟩
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
h₂ : (r - f %ₘ g).degree = (g * (q - f /ₘ g)).degree
h₄ : (r - f %ₘ g).degree < g.degree
⊢ f /ₘ g = q ∧ f %ₘ g = r
state_after: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
h₂ : (r - f %ₘ g).degree = (g * (q - f /ₘ g)).degree
h₄ : (r - f %ₘ g).degree < g.degree
h₅ : q - f /ₘ g = 0
⊢ f /ₘ g = q ∧ f %ₘ g = r
tactic: have h₅ : q - f /ₘ g = 0 :=
    _root_.by_contradiction fun hqf =>
      not_le_of_gt h₄ <|
        calc
          degree g ≤ degree g + degree (q - f /ₘ g) := by
            erw [degree_eq_natDegree hg.ne_zero, degree_eq_natDegree hqf, WithBot.coe_le_coe]
            exact Nat.le_add_right _ _
          _ = degree (r - f %ₘ g) := by rw [h₂, degree_mul']; simpa [Monic.def.1 hg]
state_before: R : Type u
S : Type v
T : Type w
A : Type z
a b : R
n : ℕ
inst✝ : Ring R
p q✝ f g q r : R[X]
hg : g.Monic
h : r + g * q = f ∧ r.degree < g.degree
a✝ : Nontrivial R
h₁ : r - f %ₘ g = -g * (q - f /ₘ g)
h₂ : (r - f %ₘ g).degree = (g * (q - f /ₘ g)).degree
h₄ : (r - f %ₘ g).degree < g.degree
h₅ : q - f /ₘ g = 0
⊢ f /ₘ g = q ∧ f %ₘ g = r
state_after: no goals
tactic: exact ⟨Eq.symm <| eq_of_sub_eq_zero h₅, Eq.symm <| eq_of_sub_eq_zero <| by simpa [h₅] using h₁⟩