Weil polynomials of small degree

Abstract

Honda and Tate showed that the isogeny classes of abelian varieties of dimension $g$ over a finite field ${\mathbb{F}}_q$ are classified in terms of $q$-Weil polynomials of degree $2g$, that is, monic integer polynomials whose set of complex roots consists of $g$ conjugate pairs of absolute value $\sqrt{q}$. There are descriptions of the space of such polynomials for $g\le 5$, but for $g=3$, $4$ and $5$, these results contain mistakes. We correct these statements. Our proofs build on a criterion that determines when a real polynomial has only real roots in terms of the non-necessarily distinct roots of its first derivative.