On $p$-adic solutions to KZ equations, ordinary crystals, and $p^s$-hypergeometric solutions

Abstract

We consider the KZ connection associated with a family of hyperelliptic curves of genus $g$ over the ring of $p$-adic integers ${\mathbb{Z}}_p$. Then the dual connection is the Gauss-Manin connection of that family. We observe that the Gauss-Manin connection has an ordinary $F$-crystal structure and its unit root subcrystal is of rank $g$. We prove that all local flat sections of the KZ connection annihilate the unite root subcrystal, and the space of all local flat sections of the KZ connection is a free ${\mathbb{Z}}_p$-module of rank $g$. We also consider the reduction modulo $p^s$ of the unit root subcrystal for any $s\ge 1$. We prove that its annihilator is generated by the so-called $p^s$-hypergeometric flat sections of the KZ connection. In particular, that means that the reduction modulo $p^s$ of an arbitrary local flat section of the KZ connection over ${\mathbb{Z}}_p$ is a linear combination of the $p^s$-hypergeometric flat sections.

12 pages, references updated, a remark added