(2,1)-quasitopos?
structures in a cohesive (∞,1)-topos
Inertia orbifold is a particular model for the free loop space object of an orbifold $X$ (or plain groupoid or smooth groupoid/stack etc.): the smooth groupoid whose objects are automorphisms in $X$ and whose morphisms are conjugation of automorphisms by morphisms in $X$. In fact for each fibered category one can construct another fibered category, its inertia and the inertia stacks are a special case of this construction.
Given a groupoid $G$ (in the category of sets) with the set of objects $G_0$ and the set of morphisms $G_1$, one defines its inertia groupoid as the groupoid whose set $S$ of objects is the set of loops, i.e. the equalizer of the source and target maps $s,t: G_1\to G_0$; and whose set of maps from $f\colon a\to a$ to $g\colon b\to b$ consists of the commutative squares with the same vertical maps of the form
i.e. of the morphisms $u \colon a\to b$ in $G_1$ such that $u^{-1}\circ g\circ u = f$.
This is isomorphic to the functor category $[S^1,G]$, where $S^1$ denotes the free groupoid on a single object with a single automorphism (equivalently, the delooping $B\mathbb{Z}$ of the integers). It is equivalent to the free loop space object of $G$ in the (2,1)-category of groupoids.
The same construction can be performed for a groupoid internal to any finitely complete category, or more generally whenever the relevant limits exist. If a (differential, topological or algebraic) stack (or, in particular, an orbifold) is represented by a groupoid, then the inertia groupoid of that groupoid represents its inertia stack. In particular, an orbifold corresponds to a Morita equivalence class of a proper étale groupoid. The inertia groupoid $\Lambda G$ of $G$ is the Morita equivalence class of the (proper étale) action groupoid for the action of $G_1$ by conjugation on the subspace $S\subset G_1$ of closed loops.
For quantum field theory on orbifolds the inertia orbifold is related to so called twisted sectors of the corresponding QFT. One can also consider more generally twisted multisectors.
At least for a finite group $G$, the groupoid convolution algebra of the inertia groupoid of $\mathbf{B}G$ is the Drinfeld double of the group convolution algebra of $G$.
Last revised on February 26, 2018 at 09:52:55. See the history of this page for a list of all contributions to it.