On rings with small Hilbert-Kunz multiplicity

Abstract

A result of Watanabe and Yoshida says that an unmixed local ring of positive characteristic is regular if and only if its Hilbert-Kunz multiplicity is one. We show that, for fixed $p$ (characteristic) and $d$ (dimension), there exist a number $\epsilon (d,p)>0$ such that any nonregular unmixed ring $R$ has Hilbert-Kunz multiplicity at least $1+\epsilon (d,p)$. We also show that local rings with sufficiently small Hilbert-Kunz multiplicity are Cohen-Macaulay and $F$-rational.

5 pages. to appear in Proc. AMS