-- Mathlib/FieldTheory/Minpoly/Basic.lean
theorem unique' {p : A[X]} (hm : p.Monic) (hp : Polynomial.aeval x p = 0)
    (hl : ∀ q : A[X], degree q < degree p → q = 0 ∨ Polynomial.aeval x q ≠ 0) :
    p = minpoly A x

Proof state:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hr : minpoly A x = p * r
hlead : 1 = r.leadingCoeff
hr0 : r ≠ 0
⊢ r.natDegree ≤ 0

Tactic:
apply_fun natDegree at hr

Result:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hlead : 1 = r.leadingCoeff
hr0 : r ≠ 0
hr : (minpoly A x).natDegree = (p * r).natDegree
⊢ r.natDegree ≤ 0

Proof state:
case inr
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : (Polynomial.aeval x) (minpoly A x %ₘ p) ≠ 0
⊢ p = minpoly A x

Tactic:
exact (h <| (aeval_modByMonic_eq_self_of_root hm hp).trans <| aeval A x).elim

Result:
no goals

Proof state:
case inl.intro
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hr : minpoly A x = p * r
hlead : 1 = r.leadingCoeff
this : r.natDegree ≤ 0
⊢ p = p * r

Tactic:
rw [eq_C_of_natDegree_le_zero this, ← Nat.eq_zero_of_le_zero this, ← leadingCoeff, ← hlead, C_1,
  mul_one]

Result:
no goals

Proof state:
case inl
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
⊢ p = minpoly A x

case inr
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : (Polynomial.aeval x) (minpoly A x %ₘ p) ≠ 0
⊢ p = minpoly A x

Tactic:
swap

Result:
case inr
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : (Polynomial.aeval x) (minpoly A x %ₘ p) ≠ 0
⊢ p = minpoly A x

case inl
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
⊢ p = minpoly A x

Proof state:
case inl.intro
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hr : minpoly A x = p * r
⊢ p = minpoly A x

Tactic:
rw [hr]

Result:
case inl.intro
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hr : minpoly A x = p * r
⊢ p = p * r

Proof state:
case h
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hlead : 1 = r.leadingCoeff
hr0 : r ≠ 0
hr : (minpoly A x).natDegree = p.natDegree + r.natDegree
⊢ ?h + r.natDegree ≤ ?h

case h
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hlead : 1 = r.leadingCoeff
hr0 : r ≠ 0
hr : (minpoly A x).natDegree = p.natDegree + r.natDegree
⊢ ℕ

Tactic:
exact hr.symm.trans_le (natDegree_le_natDegree <| min A x hm hp)

Result:
no goals

Proof state:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
hr : minpoly A x = p * 0
hlead : 1 = leadingCoeff 0
⊢ False

Tactic:
exact ne_zero hx (mul_zero p ▸ hr)

Result:
no goals

Proof state:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hlead : 1 = r.leadingCoeff
hr0 : r ≠ 0
hr : (minpoly A x).natDegree = (p * r).natDegree
⊢ r.natDegree ≤ 0

Tactic:
rw [hm.natDegree_mul' hr0] at hr

Result:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hlead : 1 = r.leadingCoeff
hr0 : r ≠ 0
hr : (minpoly A x).natDegree = p.natDegree + r.natDegree
⊢ r.natDegree ≤ 0

Proof state:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
r : A[X]
hr : minpoly A x = p * r
hlead : 1 = r.leadingCoeff
⊢ r ≠ 0

Tactic:
rintro rfl

Result:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
h : minpoly A x %ₘ p = 0
hr : minpoly A x = p * 0
hlead : 1 = leadingCoeff 0
⊢ False

Proof state:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
⊢ p = minpoly A x

Tactic:
have hx : IsIntegral A x := ⟨p, hm, hp⟩

Result:
A : Type u_1
B : Type u_2
inst✝² : CommRing A
inst✝¹ : Ring B
inst✝ : Algebra A B
x : B
p : A[X]
hm : p.Monic
hp : (Polynomial.aeval x) p = 0
hl : ∀ (q : A[X]), q.degree < p.degree → q = 0 ∨ (Polynomial.aeval x) q ≠ 0
a✝ : Nontrivial A
hx : IsIntegral A x
⊢ p = minpoly A x