Coprime-Universal Quadratic Forms

Abstract: Given a prime $p>3$, we characterize positive-definite integral quadratic forms that are coprime-universal for $p$, i.e. representing all positive integers coprime to $p$. This generalizes the $290$-Theorem by Bhargava and Hanke and extends later works by Rouse ($p=2$) and De Benedetto and Rouse ($p=3$). When $p=5,23,29,31$, our results are conditional on the coprime-universality of specific ternary forms. We prove this assumption under GRH (for Dirichlet and modular $L$-functions), following a strategy introduced by Ono and Soundararajan, together with some more elementary techniques borrowed from Kaplansky and Bhargava. Finally, we discuss briefly the problem of representing all integers in an arithmetic progression.