Differentiable groupoid objects and their abstract Lie algebroids

TL;DR

This paper generalizes the concept of Lie algebroids to a categorical setting, defining abstract Lie algebroids for groupoid objects in categories with an abstract tangent structure, broadening their applicability.

Contribution

It introduces a categorical framework for invariant vector fields and Lie brackets, defining abstract Lie algebroids in a general setting beyond classical differential geometry.

Findings

Categorical conditions for invariant vector fields

Closure under Lie bracket and Leibniz rule in the abstract setting

Examples include diffeomorphism groups and groupoids in stacks

Abstract

The infinitesimal counterpart of a Lie groupoid is its Lie algebroid. As a vector bundle, it is given by the source vertical tangent bundle restricted to the identity bisection. Its sections can be identified with the invariant vector fields on the groupoid, which are closed under the Lie bracket. We generalize this differentiation procedure to groupoid objects in any category with an abstract tangent structure in the sense of Rosick\'{y} and a scalar multiplication by a ring object that plays the role of the real numbers. We identify the categorical conditions that the groupoid object must satisfy to admit a natural notion of invariant vector fields. Then we show that invariant vector fields are closed under the Lie bracket defined by Rosick\'{y} and satisfy the Leibniz rule with respect to ring-valued morphisms on the base of the groupoid. The result is what we define axiomatically as an abstract Lie algebroid, by generalizing the underlying vector bundle to a module object in the slice category over its base. Examples include diffeomorphism groups, bisection groups of Lie groupoids, the diffeological symmetry groupoids of general relativity (Blohmann/Fernandes/Weinstein), symmetry groupoids in Lagrangian Field Theory, holonomy groupoids of singular foliations, elastic diffeological groupoids, groupoid objects in differentiable stacks, and affine groupoid schemes.