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A normal noetherian ring is a finite product of normal domains

PDF p. 9: Normal schemes and smooth-locus restriction

Statement

Assume the Axiom of Choice. Let A be a commutative noetherian ring such that every localization Ap at a prime ideal is an integrally closed domain. Then A is canonically isomorphic to the finite product of its quotients by its minimal prime ideals, and those quotients are noetherian integrally closed domains. The zero ring is the empty product.

A prime is minimal if it contains no strictly smaller prime. A domain is integrally closed if every element of its fraction field satisfying a monic polynomial over the domain belongs to the domain. A ring is noetherian if every ideal is finitely generated.

Statement references

Facts & Assumptions

[F1]

A normal noetherian ring has the prime-localization property in the statement; the zero ring satisfies it vacuously (normal noetherian ring).

[F2]

A radical ideal in a noetherian ring is the intersection of finitely many primes minimal over it; the supplier uses dependent choice, available here from the assumed Axiom of Choice (A radical ideal in a Noetherian ring is a finite intersection of minimal primes).

[A1]

The Axiom of Choice is available, including for the existence of maximal ideals containing proper ideals (The Axiom of Choice).

Proof

Given: A ring A and the Axiom of Choice as in the statement.

Goal: Obtain the stated canonical product and prove that each factor is a normal noetherian domain.

1.1A1given

If a≠0 in A, its annihilator is proper, so by [A1] it is contained in a maximal ideal m. Then a/1≠0 in Am: otherwise some s∉m annihilates a. A nilpotent element maps to zero in every prime localization because these localizations are domains. Hence A has no nonzero nilpotents, so its zero ideal is radical.

2.1F2A1step 1.1given

By [F2], 0=⋂i=1rpi for the finitely many minimal primes. Distinct pi,pj are comaximal. Otherwise their sum lies in a maximal ideal m by [A1]. Their extensions to Am are distinct minimal primes: contraction of a prime below either extension is a prime below the corresponding pi, and distinctness follows by choosing a∈pi∖pj and using primality of pj. This contradicts Am being a domain, whose only minimal prime is zero.

3.1step 2.1algebra

For each i, choose ei congruent to 1 modulo pi and to 0 modulo every other pj: multiply, over j≠i, elements of pj congruent to 1 modulo pi. Therefore A→∏iA/pi is surjective by the sum ∑ieiai, and it is injective because its kernel is the intersection in step 2.1. Each quotient is a noetherian domain. If r=0, the same kernel equality gives A=0 and the product is empty.

4.1F1A1step 3.1∎

Put B=A/pi. For every prime q of B, the corresponding localization of B is a localization of the product in step 3.1 at a prime and hence is an integrally closed domain by [F1]. If x in the fraction field of B is integral over B, then its image is integral over every Bq and therefore belongs to each. If x∉B, the ideal I={b∈B:bx∈B} is proper; choose a maximal ideal m⊇I using [A1]. The membership x∈Bm would give an s∉m with sx∈B, a contradiction. Thus B is integrally closed, completing the proof.

Depends on

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