Primary Decomposition in Boolean Rings

Abstract

Students studying the Lasker-Noether theorem on primary decomposition of ideals may want to see an example of an ideal (necessarily in a non-Noetherian ring) which does not have a primary decomposition. The most well-known counterexample is alluded to in an exercise from Atiyah and MacDonald's Commutative Algebra text. It involves the ring of continuous real-valued functions on a compact Hausdorff space, and the details require the use of Urysohn's lemma from topology. In this article, we excise the unnecessary connection to topology by finding a purely algebraic counterexample in the power set P(X) of a set X, which is a Boolean ring. Along the way we determine which principal ideals in P(X) have primary decompositions, and prove some related results about ideal decomposition in more general Boolean rings.