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Mathlib.RingTheory.Polynomial.RationalRoot

Rational root theorem and integral root theorem #

This file contains the rational root theorem and integral root theorem. The rational root theorem (num_dvd_of_is_root and den_dvd_of_is_root) for a unique factorization domain A with localization S, states that the roots of p : A[X] in A's field of fractions are of the form x / y with x y : A, x ∣ p.coeff 0 and y ∣ p.leadingCoeff. The corollary is the integral root theorem isInteger_of_is_root_of_monic: if p is monic, its roots must be integers. Finally, we use this to show unique factorization domains are integrally closed.

References #

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theorem scaleRoots_aeval_eq_zero_of_aeval_mk'_eq_zero {A : Type u_1} {S : Type u_4} [CommRing A] [CommRing S] {M : Submonoid A} [Algebra A S] [IsLocalization M S] {p : Polynomial A} {r : A} {s : ↥M} (hr : (Polynomial.aeval (IsLocalization.mk' S r s)) p = 0) :
(Polynomial.aeval ((algebraMap A S) r)) (p.scaleRoots ↑s) = 0
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theorem num_isRoot_scaleRoots_of_aeval_eq_zero {A : Type u_1} {K : Type u_2} [CommRing A] [Field K] [Algebra A K] [IsFractionRing A K] [IsDomain A] [UniqueFactorizationMonoid A] {p : Polynomial A} {x : K} (hr : (Polynomial.aeval x) p = 0) :
(p.scaleRoots ↑(IsFractionRing.den A x)).IsRoot (IsFractionRing.num A x)
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theorem num_dvd_of_is_root {A : Type u_1} {K : Type u_2} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] [Field K] [Algebra A K] [IsFractionRing A K] {p : Polynomial A} {r : K} (hr : (Polynomial.aeval r) p = 0) :
IsFractionRing.num A r ∣ p.coeff 0

Rational root theorem part 1: if r : f.codomain is a root of a polynomial over the ufd A, then the numerator of r divides the constant coefficient

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theorem den_dvd_of_is_root {A : Type u_1} {K : Type u_2} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] [Field K] [Algebra A K] [IsFractionRing A K] {p : Polynomial A} {r : K} (hr : (Polynomial.aeval r) p = 0) :
↑(IsFractionRing.den A r) ∣ p.leadingCoeff

Rational root theorem part 2: if r : f.codomain is a root of a polynomial over the ufd A, then the denominator of r divides the leading coefficient

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theorem isInteger_of_is_root_of_monic {A : Type u_1} {K : Type u_2} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] [Field K] [Algebra A K] [IsFractionRing A K] {p : Polynomial A} (hp : p.Monic) {r : K} (hr : (Polynomial.aeval r) p = 0) :
IsLocalization.IsInteger A r

Integral root theorem: if r : f.codomain is a root of a monic polynomial over the ufd A, then r is an integer

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theorem exists_integer_of_is_root_of_monic {A : Type u_1} {K : Type u_2} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] [Field K] [Algebra A K] [IsFractionRing A K] {p : Polynomial A} (hp : p.Monic) {r : K} (hr : (Polynomial.aeval r) p = 0) :
∃ (r' : A), r = (algebraMap A K) r' ∧ r' ∣ p.coeff 0
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theorem UniqueFactorizationMonoid.integer_of_integral {A : Type u_1} {K : Type u_2} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] [Field K] [Algebra A K] [IsFractionRing A K] {x : K} :
IsIntegral A x → IsLocalization.IsInteger A x
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@[instance 100]
instance UniqueFactorizationMonoid.instIsIntegrallyClosed {A : Type u_1} [CommRing A] [IsDomain A] [UniqueFactorizationMonoid A] :
IsIntegrallyClosed A