The Weil-\'etale fundamental group of a number field II

TL;DR

This paper defines and computes the Weil-étale fundamental group for number rings, establishing its properties and relation to Weil-étale cohomology, thereby advancing the understanding of arithmetic topology.

Contribution

It introduces a refined Weil-étale topos for number rings and computes its fundamental group, linking it to cohomology and confirming conjectural properties.

Findings

The Weil-étale fundamental group is a projective system of locally compact groups.

Computed Weil-étale cohomology in low degrees for number rings.

Confirmed the Weil-étale topos satisfies properties of the conjectural Lichtenbaum topos.

Abstract

We define the fundamental group underlying to Lichtenbaum's Weil-\'etale cohomology for number rings. To this aim, we define the Weil-\'etale topos as a refinement of the Weil-\'etale sites introduced in \cite{Lichtenbaum}. We show that the (small) Weil-\'etale topos of a smooth projective curve defined in this paper is equivalent to the natural definition given in \cite{Lichtenbaum-finite-field}. Then we compute the Weil-\'etale fundamental group of an open subscheme of the spectrum of a number ring. Our fundamental group is a projective system of locally compact topological groups, which represents first degree cohomology with coefficients in locally compact abelian groups. We apply this result to compute the Weil-\'etale cohomology in low degrees and to prove that the Weil-\'etale topos of a number ring satisfies the expected properties of the conjectural Lichtenbaum topos.