The asymptotic surjective number and F-surjective signature

Abstract

We study two invariants measuring the asymptotic abundance of surjections between modules. We prove that Hashimoto's asymptotic surjective number is attained over local rings, hence it is a rational number. We also prove that its associated function is constructible in families and on spectra; on the latter, it is moreover lower semicontinuous on the support of the target. We establish the existence of the F-surjective signature, a common generalization of F-signature and dual F-signature, and prove that it is superadditive in its source and varies lower semicontinuously across the spectrum under mild assumptions. As applications, we obtain new rationality results for dual F-signature and prove a transformation inequality under finite maps, with consequences for quotient singularities and reduction mod p.