Published online by Cambridge University Press: 20 November 2018
Consider a quartic $q$-Weil polynomial
$f$. Motivated by equidistribution considerations, we define, for each prime
$\ell$, a local factor that measures the relative frequency with which
$f$
$ \bmod \,\ell $ occurs as the characteristic polynomial of a symplectic similitude over
${{\mathbb{F}}_{\ell }}$. For a certain class of polynomials, we show that the resulting infinite product calculates the number of principally polarized abelian surfaces over
${{\mathbb{F}}_{q}}$ with Weil polynomial
$f$.