Classification of metric fibrations

Abstract

In this paper, we study `a fibration of metric spaces' that was originally introduced by Leinster in the study of the magnitude and called metric fibrations. He showed that the magnitude of a metric fibration splits into the product of those of the fiber and the base, which is analogous to the Euler characteristic and topological fiber bundles. His idea and our approach is based on Lawvere's suggestion of viewing a metric space as an enriched category. Actually, the metric fibration turns out to be the restriction of the enriched Grothendieck fibrations to metric spaces. We give a complete classification of metric fibrations by several means, which is parallel to that of topological fiber bundles. That is, the classification of metric fibrations is reduced to that of `principal fibrations', which is done by the `1-Cech cohomology' in an appropriate sense. Here we introduce the notion of torsors in the category of metric spaces, and the discussions are analogous to the sheaf theory. Further, we can define the `fundamental group $π^m_1(X)$' of a metric space $X$, which is a group object in metric spaces, such that the conjugation classes of homomorphisms $Hom(π^m_1(X), G)$ corresponds to the isomorphism classes of `principal $G$-fibrations' over $X$. Namely, it is classified like topological covering spaces.