Let $[X,lambda]$ be a principally polarized abelian variety over a finite field with commutative endomorphism ring; further suppose that either $X$ is ordinary or the field is prime. Motivated by an equidistribution heuristic, we introduce a factor $ u_v([X,lambda])$ for each place $v$ of $mathbb Q$, and show that the product of these factors essentially computes the size of the isogeny class of $[X,lambda]$. The derivation of this mass formula depends on a formula of Kottwitz and on analysis of measures on the group of symplectic similitudes, and in particular does not rely on a calculation of class numbers.
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