Arithmetic statistics of Prym surfaces

Jef Laga

doi:10.1007/s00208-022-02398-5

Arithmetic statistics of Prym surfaces

Math Ann. 2023;386(1-2):247-327. doi: 10.1007/s00208-022-02398-5. Epub 2022 Apr 12.

Author

Jef Laga¹

Affiliation

¹ Department of Pure Mathematics and Mathematical Statistics, University of Cambridge, Wilberforce Road, Cambridge, CB3 0WB UK.

Abstract

We consider a family of abelian surfaces over $Q$ arising as Prym varieties of double covers of genus-1 curves by genus-3 curves. These abelian surfaces carry a polarization of type (1, 2) and we show that the average size of the Selmer group of this polarization equals 3. Moreover we show that the average size of the 2-Selmer group of the abelian surfaces in the same family is bounded above by 5. This implies an upper bound on the average rank of these Prym varieties, and gives evidence for the heuristics of Poonen and Rains for a family of abelian varieties which are not principally polarized. The proof is a combination of an analysis of the Lie algebra embedding $F_{4} \subset E_{6}$ , invariant theory, a classical geometric construction due to Pantazis, a study of Néron component groups of Prym surfaces and Bhargava's orbit-counting techniques.

Keywords: 11G10; 14G25; 20G30.