# Answer to Question #17369 in Algebra for Melvin Henriksen

Question #17369
For a commutative ring R, show that the following are equivalent: (1) R has Krull dimension 0. (2) rad R is nil and R/rad R is von Neumann regular. (3) For any a &isin; R, the descending chain Ra &supe; Ra2 &supe; . . . stabilizes. (4) For any a &isin; R, there exists n &ge; 1 such that an is regular.
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2012-10-31T08:55:07-0400
(1) &rArr; (2). By (1), rad R is the intersection of all prime ideals, so itis nil. For the rest, we may assume that rad R = 0. For any a &isin; R, it suffices to show that Ra = Ra2.Let p be any prime ideal of R. Since R is reduced, so is Rp.But pRp is the only prime ideal of Rp, so we have pRp = 0.Therefore, Rp is a field. In particular, Rpa = Rpa2.It follows that (Ra/Ra2)p = 0 for every prime ideal p &sub; R; hence Ra = Ra2, asdesired.
(2) &rArr; (3). Let a &isin; R. By (2 ),a = a2b &isin; R/rad R for some b &isin; R, so (a &minus; a2b)n =0 for some n &ge; 1. Expanding the LHS and transposing, we get an&isin; Ran+1, and hence Ran = Ran+1= &middot; &middot; &middot; .
(3) &rArr; (4) is clear.
(4) &rArr; (1). Let p be any prime ideal, and a is not in p. By (4), an= a2nb for some b &isin; R and some n &ge; 1. Then an(1&minus; anb) = 0 implies that 1 &minus; anb &isin; p. This shows that R/p is a field, so p is a maximal ideal.

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