$F$-signature under birational morphisms

Abstract

We study $F$-signature under proper birational morphisms $\pi :Y\to X$, showing that $F$-signature strictly increases for small morphisms or if $K_Y\ge {\pi }^{\ast }{K}_X$. In certain cases, we can even show that the $F$-signature of $Y$ is at least twice as that of $X$. We also provide examples of $F$-signature dropping and Hilbert-Kunz multiplicity increasing under birational maps without these hypotheses.

15 pages, comments welcome