nLab connected category




A groupoid is connected if it is inhabited and every object is connected by a morphism to every other object.

So the skeletal connected groupoids are precisely the delooping groupoids of groups.

In homotopy theory, groupoids model exactly the homotopy 1-types and connected groupoids model the connected homotopy 1-types. For instance the fundamental groupoid of a connected topological space is a connected groupoid.

Every category CC induces a free groupoid F(C)F(C) by freely inverting all its morphisms. A category is called connected if the groupoid F(C)F(C) is.


A category CC is connected if it is inhabited and the following equivalent conditions hold:

Note that the empty category is not connected. For other purposes, one can argue about whether the empty set should be called “connected” (see connected space), but for the applications of connected categories, the empty category should definitely not be called connected. In particular, a terminal object is not a connected limit.


The notion of connected groupoids was originally defined in

whence some authors also speak of Brandt groupoids.

For more see the references at groupoids.

Last revised on June 6, 2023 at 15:46:59. See the history of this page for a list of all contributions to it.