# Loop space

In topology, a branch of mathematics, the **loop space** Ω*X* of a pointed topological space *X* is the space of (based) loops in *X*, i.e. continuous pointed maps from the pointed circle *S*^{1} to *X*, equipped with the compact-open topology. Two loops can be multiplied by concatenation. With this operation, the loop space is an *A*_{∞}-space. That is, the multiplication is homotopy coherently associative.

The set of path components of Ω*X*, i.e. the set of based-homotopy equivalence classes of based loops in *X*, is a group, the fundamental group *π*_{1}(*X*).

The **iterated loop spaces** of *X* are formed by applying Ω a number of times.

There is an analogous construction for topological spaces without basepoint. The **free loop space** of a topological space *X* is the space of maps from the circle *S*^{1} to *X* with the compact-open topology. The free loop space of *X* is often denoted by .

As a functor, the free loop space construction is right adjoint to cartesian product with the circle, while the loop space construction is right adjoint to the reduced suspension. This adjunction accounts for much of the importance of loop spaces in stable homotopy theory. (A related phenomenon in computer science is currying, where the cartesian product is adjoint to the hom functor.) Informally this is all referred to as Eckmann–Hilton duality.

## Eckmann–Hilton duality

The loop space is dual to the suspension of the same space; this duality is sometimes called Eckmann–Hilton duality. The basic observation is that

where is the set of homotopy classes of maps , and is the suspension of A, and denotes the natural homeomorphism. This homeomorphism is essentially that of currying, modulo the quotients needed to convert the products to reduced products.

In general, does not have a group structure for arbitrary spaces and . However, it can be shown that and do have natural group structures when and are pointed, and the aforesaid isomorphism is of those groups.[1] Thus, setting (the sphere) gives the relationship

- .

This follows since the homotopy group is defined as and the spheres can be obtained via suspensions of each-other, i.e. .[2]

## See also

## References

- May, J. P. (1999),
*A Concise Course in Algebraic Topology*(PDF), U. Chicago Press, Chicago, retrieved 2016-08-27*(See chapter 8, section 2)* - Topospaces wiki - Loop space of a based topological space

- Adams, John Frank (1978),
*Infinite loop spaces*, Annals of Mathematics Studies,**90**, Princeton University Press, ISBN 978-0-691-08207-3, MR 0505692 - May, J. Peter (1972),
*The Geometry of Iterated Loop Spaces*, Berlin, New York: Springer-Verlag, doi:10.1007/BFb0067491, ISBN 978-3-540-05904-2, MR 0420610