Kummer-Dedekind theorem

This file proves the monogenic version of the Kummer-Dedekind theorem on the splitting of prime ideals in an extension of the ring of integers. This states that if I is a prime ideal of Dedekind domain R and S = R[α] for some α that is integral over R with minimal polynomial f, then the prime factorisations of I * S and f mod I have the same shape, i.e. they have the same number of prime factors, and each prime factors of I * S can be paired with a prime factor of f mod I in a way that ensures multiplicities match (in fact, this pairing can be made explicit with a formula).

Main definitions

• normalizedFactorsMapEquivNormalizedFactorsMinPolyMk : The bijection in the Kummer-Dedekind theorem. This is the pairing between the prime factors of I * S and the prime factors of f mod I.

Main results

• normalized_factors_ideal_map_eq_normalized_factors_min_poly_mk_map : The Kummer-Dedekind theorem.
• Ideal.irreducible_map_of_irreducible_minpoly : I.map (algebraMap R S) is irreducible if (map (Ideal.Quotient.mk I) (minpoly R pb.gen)) is irreducible, where pb is a power basis of S over R.

TODO

• Prove the Kummer-Dedekind theorem in full generality.

• Prove the converse of Ideal.irreducible_map_of_irreducible_minpoly.

• Prove that normalizedFactorsMapEquivNormalizedFactorsMinPolyMk can be expressed as normalizedFactorsMapEquivNormalizedFactorsMinPolyMk g = ⟨I, G(α)⟩ for g a prime factor of f mod I and G a lift of g to R[X].

References

• [J. Neukirch, Algebraic Number Theory][Neukirch1992]

Tags

kummer, dedekind, kummer dedekind, dedekind-kummer, dedekind kummer

def conductor (R : Type u_1) {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] (x : S) : Ideal S

Let S / R be a ring extension and x : S, then the conductor of R is the biggest ideal of S contained in R.

theorem conductor_eq_of_eq {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {y : S} (h : ↑(Algebra.adjoin R {x}) = ↑(Algebra.adjoin R {y})) : conductor R x = conductor R y

theorem conductor_subset_adjoin {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} : ↑(conductor R x) ⊆ ↑(Algebra.adjoin R {x})

theorem mem_conductor_iff {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {y : S} : y ∈ conductor R x ↔ ∀ (b : S), y * b ∈ Algebra.adjoin R {x}

theorem conductor_eq_top_of_adjoin_eq_top {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} (h : Algebra.adjoin R {x} = ⊤) : conductor R x = ⊤

theorem conductor_eq_top_of_powerBasis {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] (pb : PowerBasis R S) : conductor R pb.gen = ⊤

theorem prod_mem_ideal_map_of_mem_conductor {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} {p : R} {z : S} (hp : p ∈ Ideal.comap (algebraMap R S) (conductor R x)) (hz' : z ∈ Ideal.map (algebraMap R S) I) : ↑(algebraMap R S) p * z ∈ ↑(algebraMap { x // x ∈ Algebra.adjoin R {x} } S) '' ↑(Ideal.map (algebraMap R { x // x ∈ Algebra.adjoin R {x} }) I)

This technical lemma tell us that if C is the conductor of R and I is an ideal of R then p * (I * S) ⊆ I * R for any p in C ∩ R

theorem comap_map_eq_map_adjoin_of_coprime_conductor {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (h_alg : Function.Injective ↑(algebraMap { x // x ∈ Algebra.adjoin R {x} } S)) : Ideal.comap (algebraMap { x // x ∈ Algebra.adjoin R {x} } S) (Ideal.map (algebraMap R S) I) = Ideal.map (algebraMap R { x // x ∈ Algebra.adjoin R {x} }) I

A technical result telling us that (I * S) ∩ R = I * R for any ideal I of R.

noncomputable def quotAdjoinEquivQuotMap {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (h_alg : Function.Injective ↑(algebraMap { x // x ∈ Algebra.adjoin R {x} } S)) : { x // x ∈ Algebra.adjoin R {x} } ⧸ Ideal.map (algebraMap R { x // x ∈ Algebra.adjoin R {x} }) I ≃+* S ⧸ Ideal.map (algebraMap R S) I

The canonical morphism of rings from R ⧸ (I*R) to S ⧸ (I*S) is an isomorphism when I and (conductor R x) ∩ R are coprime.

@[simp]

theorem quotAdjoinEquivQuotMap_apply_mk {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (h_alg : Function.Injective ↑(algebraMap { x // x ∈ Algebra.adjoin R {x} } S)) (a : { x // x ∈ Algebra.adjoin R {x} }) : ↑(quotAdjoinEquivQuotMap hx h_alg) (↑(Ideal.Quotient.mk (Ideal.map (algebraMap R { x // x ∈ Algebra.adjoin R {x} }) I)) a) = ↑(Ideal.Quotient.mk (Ideal.map (algebraMap R S) I)) ↑a

noncomputable def KummerDedekind.normalizedFactorsMapEquivNormalizedFactorsMinPolyMk {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} [IsDomain R] [IsIntegrallyClosed R] [IsDomain S] [IsDedekindDomain S] [NoZeroSMulDivisors R S] (hI : Ideal.IsMaximal I) (hI' : I ≠ ⊥) (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (hx' : IsIntegral R x) : ↑{J | J ∈ UniqueFactorizationMonoid.normalizedFactors (Ideal.map (algebraMap R S) I)} ≃ ↑{d | d ∈ UniqueFactorizationMonoid.normalizedFactors (Polynomial.map (Ideal.Quotient.mk I) (minpoly R x))}

The first half of the Kummer-Dedekind Theorem in the monogenic case, stating that the prime factors of I*S are in bijection with those of the minimal polynomial of the generator of S over R, taken mod I.

theorem KummerDedekind.multiplicity_factors_map_eq_multiplicity {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} [IsDomain R] [IsIntegrallyClosed R] [IsDomain S] [IsDedekindDomain S] [NoZeroSMulDivisors R S] (hI : Ideal.IsMaximal I) (hI' : I ≠ ⊥) (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (hx' : IsIntegral R x) {J : Ideal S} (hJ : J ∈ UniqueFactorizationMonoid.normalizedFactors (Ideal.map (algebraMap R S) I)) : multiplicity J (Ideal.map (algebraMap R S) I) = multiplicity (↑(↑(KummerDedekind.normalizedFactorsMapEquivNormalizedFactorsMinPolyMk hI hI' hx hx') { val := J, property := hJ })) (Polynomial.map (Ideal.Quotient.mk I) (minpoly R x))

The second half of the Kummer-Dedekind Theorem in the monogenic case, stating that the bijection FactorsEquiv' defined in the first half preserves multiplicities.

theorem KummerDedekind.normalizedFactors_ideal_map_eq_normalizedFactors_min_poly_mk_map {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} [IsDomain R] [IsIntegrallyClosed R] [IsDomain S] [IsDedekindDomain S] [NoZeroSMulDivisors R S] (hI : Ideal.IsMaximal I) (hI' : I ≠ ⊥) (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (hx' : IsIntegral R x) : UniqueFactorizationMonoid.normalizedFactors (Ideal.map (algebraMap R S) I) = Multiset.map (fun f => ↑(↑(KummerDedekind.normalizedFactorsMapEquivNormalizedFactorsMinPolyMk hI hI' hx hx').symm f)) (Multiset.attach (UniqueFactorizationMonoid.normalizedFactors (Polynomial.map (Ideal.Quotient.mk I) (minpoly R x))))

The Kummer-Dedekind Theorem.

theorem KummerDedekind.Ideal.irreducible_map_of_irreducible_minpoly {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S] {x : S} {I : Ideal R} [IsDomain R] [IsIntegrallyClosed R] [IsDomain S] [IsDedekindDomain S] [NoZeroSMulDivisors R S] (hI : Ideal.IsMaximal I) (hI' : I ≠ ⊥) (hx : Ideal.comap (algebraMap R S) (conductor R x) ⊔ I = ⊤) (hx' : IsIntegral R x) (hf : Irreducible (Polynomial.map (Ideal.Quotient.mk I) (minpoly R x))) : Irreducible (Ideal.map (algebraMap R S) I)