Relative fundamental groups and rational points

Abstract

In this paper we define a relative rigid fundamental group, which associates to a section $p$ of a smooth and proper morphism $f:X\to S$ in characteristic $p$, a Hopf algebra in the ind-category of overconvergent $F$-isocrystals on $S$. We prove a base change property, which says that the fibres of this object are the Hopf algebras of the rigid fundamental groups of the fibres of $f$. We explain how to use this theory to define period maps as Kim does for varieties over number fields, and show in certain cases that the targets of these maps can be interpreted as varieties.