We study the behavior of Hilbert-Kunz multiplicity for powers of an ideal,
especially the case of stable ideals, and ideals in local rings of dimensio
n 2. We can characterize regular local rings by certain equality between Hi
lbert-Kunz multiplicity and usual multiplicity.
We show that rings with "minimal" Hilbert-Kunz multiplicity relative to usu
al multiplicity are "Veronoese subrings" in dimension 2.