On the sparsity of positive-definite automorphic forms within a family

Abstract

Baker and Montgomery have proved that almost all Fekete polynomials with respect to a certain ordering have at least one zero on the interval (0, 1). It is also known that a Fekete polynomial has no zeros on the interval (0, 1) if and only if the corresponding automorphic form is positive-definite. Generalizing their result, we formulate an axiomatic result about sets of automorphic forms π satisfying certain averages when suitably ordered to ensure that almost all p’s are not positive-definite within such sets. We then apply the result to various families, including the family of holomorphic cusp forms, the family of Hilbert class characters of imaginary quadratic fields, and the family of elliptic curves. In the appendix, we apply the result to general families of automorphic forms defined by Sarnak, Shin, and Templer.