Deducing information about curves over finite fields from their Weil polynomials

Everett W. Howe

Published 2021-10-08, updated 2022-09-03Version 2

We discuss methods for using the Weil polynomial of an isogeny class of abelian varieties over a finite field to determine properties of the curves (if any) whose Jacobians lie in the isogeny class. Some methods are strong enough to show that there are no curves with the given Weil polynomial, while other methods can sometimes be used to show that a curve with the given Weil polynomial must have nontrivial automorphisms, or must come provided with a map of known degree to an elliptic curve with known trace. Such properties can sometimes lead to efficient methods for searching for curves with the given Weil polynomial. Many of the techniques we discuss were inspired by methods that Serre used in his 1985 Harvard class on rational points on curves over finite fields. The recent publication of the notes for this course gives an incentive for reviewing the developments in the field that have occurred over the intervening years.