The defect of the F-pure threshold

Abstract

Introduced by Takagi and Watanabe, the F-pure threshold is an invariant defined in terms of the Frobenius homomorphism. While it finds applications in various settings, it is primarily used as a local invariant. The purpose of this note is to start filling this gap by opening the study of its behavior on a scheme. To this end, we define the defect of the F-pure threshold of a local ring $(R,\mathfrak{m})$ by setting $\mathrm{dfpt}(R)=\dim (R)-\mathrm{fpt}(\mathfrak{m})$. It turns out that this invariant defines an upper semi-continuous function on a scheme and satisfies Bertini-type theorems. We also study the behavior of the defect of the F-pure threshold under flat extensions and after blowing up the maximal ideal of a local ring.

v3: in v2 and in the published version of the proof of Lemma 4.6, it is claimed that A is Gorenstein. However, in our assumptions one can only say that A is quasi-Gorenstein; this correction does not affect the rest of the proof