-- Mathlib/RingTheory/Polynomial/GaussLemma.lean
theorem Monic.dvd_of_fraction_map_dvd_fraction_map [IsIntegrallyClosed R] {p q : R[X]}
    (hp : p.Monic) (hq : q.Monic)
    (h : q.map (algebraMap R K) ∣ p.map (algebraMap R K)) : q ∣ p

Proof state:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' * C r.leadingCoeff = r
⊢ q ∣ p

Tactic:
rw [Monic.leadingCoeff, C_1, mul_one] at hr'

Result:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' = r
⊢ q ∣ p

case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' * C r.leadingCoeff = r
⊢ r.Monic

Proof state:
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
h : Polynomial.map (algebraMap R K) q ∣ Polynomial.map (algebraMap R K) p
⊢ q ∣ p

Tactic:
obtain ⟨r, hr⟩ := h

Result:
case intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
⊢ q ∣ p

Proof state:
case intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
⊢ q ∣ p

Tactic:
obtain ⟨d', hr'⟩ := IsIntegrallyClosed.eq_map_mul_C_of_dvd K hp (dvd_of_mul_left_eq _ hr.symm)

Result:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' * C r.leadingCoeff = r
⊢ q ∣ p

Proof state:
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' * C r.leadingCoeff = r
⊢ (Polynomial.map (algebraMap R K) q * r).Monic

Tactic:
simpa [← hr] using hp.map _

Result:
no goals

Proof state:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
d' : R[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) (q * d')
hr' : Polynomial.map (algebraMap R K) d' = r
⊢ q ∣ p

Tactic:
exact dvd_of_mul_right_eq _ (Polynomial.map_injective _ (IsFractionRing.injective R K) hr.symm)

Result:
no goals

Proof state:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' = r
⊢ q ∣ p

Tactic:
rw [← hr', ← Polynomial.map_mul] at hr

Result:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
d' : R[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) (q * d')
hr' : Polynomial.map (algebraMap R K) d' = r
⊢ q ∣ p

Proof state:
case intro.intro
R : Type u_1
inst✝⁴ : CommRing R
K : Type u_2
inst✝³ : Field K
inst✝² : Algebra R K
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
p q : R[X]
hp : p.Monic
hq : q.Monic
r : K[X]
hr : Polynomial.map (algebraMap R K) p = Polynomial.map (algebraMap R K) q * r
d' : R[X]
hr' : Polynomial.map (algebraMap R K) d' * C r.leadingCoeff = r
⊢ r.Monic

Tactic:
exact Monic.of_mul_monic_left (hq.map (algebraMap R K)) (by simpa [← hr] using hp.map _)

Result:
no goals