nLab
empty bundle

Context

Bundles

Contents

Idea

In as far as a general bundle over some base object BB is a morphism EBE \to B into BB, out of some “total space” object EE, the empty bundle over BB is the one whose total space is “empty”, hence whose projection map is the empty function.

Definition

In TopologicalSpaces an empty bundle has the empty topological space as its total space, while in SimplicialSets the empty bundle has the the empty simplicial set as its total space, etc.:

B. \array{ \varnothing \\ \big\downarrow \\ B \mathrlap{\,.} }

Generally, one may speak of empty bundles internal to any ambient category in which the initial object is strict (e.g. any topos) in that every morphism to the initial object is an isomorphism, so that

(1)(X)X. \exists \big( X \overset{}{\rightarrow} \varnothing \big) \;\;\;\;\; \Leftrightarrow \;\;\;\;\; X \simeq \varnothing \,.

Properties

In topological spaces

In TopologicalSpaces, any empty bundle

In simplicial sets

Similarly, in SimplicialSets every empty bundle

Application

In equivariant bundle theory

Empty fiber bundles play a central role in the context of equivariant bundles, where they frequently appear as fixed loci of non-empty bundles.