Published online by Cambridge University Press: 13 February 2013
We describe a probability distribution on isomorphism classes of principally quasi-polarized $p$-divisible groups over a finite field
$k$ of characteristic
$p$ which can reasonably be thought of as a ‘uniform distribution’, and we compute the distribution of various statistics (
$p$-corank,
$a$-number, etc.) of
$p$-divisible groups drawn from this distribution. It is then natural to ask to what extent the
$p$-divisible groups attached to a randomly chosen hyperelliptic curve (respectively, curve; respectively, abelian variety) over
$k$ are uniformly distributed in this sense. This heuristic is analogous to conjectures of Cohen–Lenstra type for
$\text{char~} k\not = p$, in which case the random
$p$-divisible group is defined by a random matrix recording the action of Frobenius. Extensive numerical investigation reveals some cases of agreement with the heuristic and some interesting discrepancies. For example, plane curves over
${\mathbf{F} }_{3} $ appear substantially less likely to be ordinary than hyperelliptic curves over
${\mathbf{F} }_{3} $.