Inner automorphisms of groupoids: In memory of Pieter Hofstra

Bergman has given the following abstract characterisation of the inner automorphisms of a group G: they are exactly those automorphisms of G which can be extended functorially along any homomorphism G → H to an automorphism of H. This leads naturally to a definition of “inner automorphism” applicable to the objects of any category. Bergman and Hofstra–Parker–Scott have computed these inner automorphisms for various structures including k-algebras, monoids, lattices, unital rings, and quandles—showing that, in each case, they are given by an obvious notion of conjugation.

In this paper, we compute the inner automorphisms of groupoids, showing that they are exactly the automorphisms induced by conjugation by a bisection. The twist is that this result is false in the category of groupoids and functors; to make it true, we must instead work with the less familiar category of groupoids and cofunctors in the sense of Higgins and Mackenzie. Besides our main result, we also discuss generalisations to topological and Lie groupoids, to categories and to partial automorphisms, and examine the link with the theory of inverse semigroups.