Isotrivial elliptic surfaces in positive characteristic

Abstract

We study relatively minimal surfaces equipped with a strongly isotrivial elliptic fibration in positive characteristic by means of the notion of equivariantly normal curves introduced and developed recently by Brion in [Pure Appl. Math. Q. 20 (2024), 1065–1095 and arXiv:2405.12020v1]. Such surfaces are isomorphic to a contracted product $E{\times }^{G}X$, where $E$ is an elliptic curve, $G$ is a finite subgroup scheme of $E$ and $X$ is a $G$-normal curve. Using this description, we compute their Betti numbers to determine their birational classes. This allows us to complete the classification of maximal automorphism groups of surfaces in any characteristic, extending the result in characteristic zero obtained in [Ann. Inst. Fourier (Grenoble) 74 (2024), 545–587]. When $G$ is diagonalizable, we compute additional invariants to study the structure of their Picard schemes.