Weight decompositions on etale fundamental groups

Research paper by J. P. Pridham

Indexed on: 03 Jul '07Published on: 03 Jul '07Published in: Mathematics - Algebraic Geometry


Let X be a smooth or proper variety defined over a finite field. The geometric etale fundamental group of X is a normal subgroup of the Weil group, so conjugation gives it a Weil action. We consider the pro-Q_l-algebraic completion of the fundamental group as a non-abelian Weil representation. Lafforgue's Theorem and Deligne's Weil II theorems imply that this affine group scheme is mixed, in the sense that its structure sheaf is a mixed Weil representation. When X is smooth, weight restrictions apply, affecting the possibilities for the structure of this group. This gives new examples of groups which cannot arise as etale fundamental groups of smooth varieties.