Tangent spaces and tangent bundles for diffeological spaces

TL;DR

This paper extends the concept of tangent spaces and bundles from smooth manifolds to diffeological spaces, addressing definitions, properties, and examples including singular and infinite-dimensional spaces.

Contribution

It compares two tangent space definitions, identifies issues with existing tangent bundle definitions, and proposes an improved, smooth-compatible tangent bundle construction for diffeological spaces.

Findings

Internal and external tangent spaces often differ in general diffeological spaces.

Existing tangent bundle definitions can have non-smooth operations, which we correct with a new dvs diffeology.

The improved tangent bundle behaves well in various examples, including singular and infinite-dimensional spaces.

Abstract

We study how the notion of tangent space can be extended from smooth manifolds to diffeological spaces, which are generalizations of smooth manifolds that include singular spaces and infinite-dimensional spaces. We focus on two definitions. The internal tangent space of a diffeological space is defined using smooth curves into the space, and the external tangent space is defined using smooth derivations on germs of smooth functions. We prove fundamental results about these tangent spaces, compute them in many examples, and observe that while they agree for smooth manifolds and many of the examples, they do not agree in general. After this, we recall Hector's definition of the tangent bundle of a diffeological space, and show that both scalar multiplication and addition can fail to be smooth, revealing errors in several references. We then give an improved definition of the tangent bundle, using what we call the dvs diffeology, which ensures that scalar multiplication and addition are smooth. We establish basic facts about these tangent bundles, compute them in many examples, and study the question of whether the fibres of tangent bundles are fine diffeological vector spaces. Our examples include singular spaces, spaces whose natural topology is non-Hausdorff (e.g., irrational tori), infinite-dimensional vector spaces and diffeological groups, and spaces of smooth maps between smooth manifolds (including diffeomorphism groups).