Loop group

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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 mapsS^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]


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


  • 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 

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