The monodromy of cyclic Pryms
Eric M. Rains
TL;DR
The paper analyzes the monodromy of Pryms arising from cyclic covers, showing that for broad one-parameter families the geometric monodromy is large, sandwiched between unitary groups and their derived subgroups, by connecting period integrals to Jordan-Pochhammer monodromy and employing a detailed prime-by-prime largeness and p-adic lifting strategy. It then extends the analysis to singular curves, higher-genus bases, and finite characteristic, using degenerations, ramification data control, and p-adic Lie algebra techniques to propagate largeness across settings. A key application links Prym monodromy to Selmer groups of elliptic surfaces with j=0 or j=1728, demonstrating that these Selmer groups can violate standard heuristics and yielding explicit average sizes in certain arithmetic regimes. Overall, the work provides a robust framework for understanding monodromy in Prym-related families and translates these geometric results into arithmetic consequences for Selmer groups.
Abstract
The {\em Prym} of a cyclic covering of smooth projective curves is the ``new'' part of the Jacobian: the quotient of the Jacobian of the covering curve by the Jacobians of the intermediate covers. Given a family of such coverings, the fundamental group of the base of the family acts on the Tate modules of the Pryms, and the image of this representation is a key ingredient in answering arithmetic statistics questions about the distribution of the group structure of the $L$-torsion of a random Prym in the family. (Over ${\mathbb{F}}_q$, the action of Frobenius is roughly uniformly distributed over the {\em arithmetic} monodromy, a coset of the image of the fundamental group of the base change to $\bar{\mathbb{F}}_q$ (the {\em geometric} monodromy).) In the present note, we show for a number of natural families that (with limited exceptions) the geometric monodromy is sandwiched between a certain unitary group and its derived subgroup. In particular, this holds for the one-parameter families obtained by starting with any fixed cover and varying one (tame) ramification point. As an application, we deduce analogous largeness results for the monodromy of the Selmer groups of elliptic surfaces with $j=0$ or $j=1728$, by relating them to cyclic covers of degree 6 or 4 respectively, implying that their Selmer groups do not satisfy the standard heuristics. For instance, for eliptic surfaces with $j=0$ of sufficiently large height over ${\mathbb{P}}^1_{\mathbb{F}_q}$, the average size of the $l$-Selmer group is $l+3+o_q(1)$ when $l$ (fixed) and $q$ (large) are both 1 mod 3, compared to $l+1+o_q(1)$ for general elliptic surfaces.