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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 be a commutative noetherian ring such that every localization at a prime ideal is an integrally closed domain. Then 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
A normal noetherian ring has the prime-localization property in the statement; the zero ring satisfies it vacuously (normal noetherian ring).
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).
The Axiom of Choice is available, including for the existence of maximal ideals containing proper ideals (The Axiom of Choice).
Proof
Given: A ring 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.
If in , its annihilator is proper, so by [A1] it is contained in a maximal ideal . Then in : otherwise some annihilates . A nilpotent element maps to zero in every prime localization because these localizations are domains. Hence has no nonzero nilpotents, so its zero ideal is radical.
By [F2], for the finitely many minimal primes. Distinct are comaximal. Otherwise their sum lies in a maximal ideal by [A1]. Their extensions to are distinct minimal primes: contraction of a prime below either extension is a prime below the corresponding , and distinctness follows by choosing and using primality of . This contradicts being a domain, whose only minimal prime is zero.
For each , choose congruent to modulo and to modulo every other : multiply, over , elements of congruent to modulo . Therefore is surjective by the sum , and it is injective because its kernel is the intersection in step 2.1. Each quotient is a noetherian domain. If , the same kernel equality gives and the product is empty.
Put . For every prime of , the corresponding localization of is a localization of the product in step 3.1 at a prime and hence is an integrally closed domain by [F1]. If in the fraction field of is integral over , then its image is integral over every and therefore belongs to each. If , the ideal is proper; choose a maximal ideal using [A1]. The membership would give an with , a contradiction. Thus is integrally closed, completing the proof.
Depends on
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