# Loop group

For groups of actors involved in re-recording movie dialogue during post-production (commonly known in the entertainment industry as "loop groups"), see Dubbing (filmmaking).

In mathematics, a loop group is a group of loops in a topological group G with multiplication defined pointwise. Specifically, let LG denote the space of continuous maps$S^1 \to G$ equipped with the compact-open topology. An element of $LG$ is called a loop in G. Pointwise multiplication of such loops gives $LG$ the structure of a topological group. The space $LG$ is called the free loop group on $G$. A loop group is any subgroup of the free loop group $LG$.

An important example of a loop group is the group

$\Omega G \,$

of based loops on $G$. It is defined to be the kernel of the evaluation map

$e_1: LG \to G$,

and hence is a closed normal subgroup of $LG$. (Here, $e_1$ is the map that sends a loop to its value at $1$.) Note that we may embed $G$ into $LG$ as the subgroup of constant loops. Consequently, we arrive at a split exact sequence

$1\to \Omega G \to LG \to G\to 1$.

The space $LG$ splits as a semi-direct product,

$LG = \Omega G \rtimes G$.

We may also think of $\Omega G$ as the loop space on $G$. From this point of view, $\Omega G$ is an H-space with respect to concatenation of loops. On the face of it, this seems to provide $\Omega G$ with two very different product maps. However, it can be shown that concatenation and pointwise multiplication are homotopic. Thus, in terms of the homotopy theory of $\Omega G$, these maps are interchangeable.

Loop groups were used to explain the phenomenon of Bäcklund transforms in soliton equations by Chuu-Lian Terng and Karen Uhlenbeck.[1]

## Notes

1. ^ Geometry of Solitons by Chuu-Lian Terng and Karen Uhlenbeck

## References

• Pressley, Andrew; Segal, Graeme (1986), Loop groups, Oxford Mathematical Monographs. Oxford Science Publications, The Clarendon Press, Oxford University Press, New York, ISBN 0-19-853535-X, MR 0900587