Hilbert-Kunz multiplicity of products of ideals

Abstract

We give bounds for the Hilbert-Kunz multiplicity of the product of two ideals, and we characterize the equality in terms of the tight closures of the ideals. Connections are drawn with $\ast$-spread and with ordinary length calculations.

Since version 1, we cleaned up the exposition in the introduction and removed extraneous material from the body of the text. 9 pages, comments welcome