# Suspension (topology)

In topology, the **suspension** of a topological space *X* is intuitively obtained by stretching *X* into a cylinder and then collapsing both end faces to points. One views *X* as "suspended" between these end points.

The space *SX* is sometimes called the **unreduced**, **unbased**, or **free suspension** of *X*, to distinguish it from the **reduced suspension** Σ*X* of a pointed space described below.

The reduced suspension can be used to construct a homomorphism of homotopy groups, to which the Freudenthal suspension theorem applies. In homotopy theory, the phenomena which are preserved under suspension, in a suitable sense, make up stable homotopy theory.

## Definition and properties of suspension

Given a topological space *X*, the suspension of *X* is defined as

the quotient space of the product of *X* with the unit interval *I* = [0, 1] modulo the equivalence relation generated by

One can view the suspension as two cones on *X* glued together at their base; it is also homeomorphic to the join where is a discrete space with two points.

In rough terms *S* increases the dimension of a space by one: it takes an *n*-sphere to an (*n* + 1)-sphere for *n* ≥ 0.

Given a continuous map there is a continuous map defined by where square brackets denote equivalence classes. This makes into a functor from the category of topological spaces to itself.

## Reduced suspension

If *X* is a pointed space (with basepoint *x*_{0}), there is a variation of the suspension which is sometimes more useful. The **reduced suspension** or **based suspension** Σ*X* of *X* is the quotient space:

- .

This is the equivalent to taking *SX* and collapsing the line (*x*_{0} × *I*) joining the two ends to a single point. The basepoint of the pointed space Σ*X* is taken to be the equivalence class of (*x*_{0}, 0).

One can show that the reduced suspension of *X* is homeomorphic to the smash product of *X* with the unit circle *S*^{1}.

For well-behaved spaces, such as CW complexes, the reduced suspension of *X* is homotopy equivalent to the unbased suspension.

### Adjunction of reduced suspension and loop space functors

Σ gives rise to a functor from the category of pointed spaces to itself. An important property of this functor is that it is left adjoint to the functor taking a pointed space to its loop space . In other words, we have a natural isomorphism

where and are pointed spaces and stands for continuous maps that preserve basepoints. This adjunction can be understood geometrically, as follows. arises out of if a pointed circle is attached to every non-basepoint of , and the basepoints of all these circles are identified and glued to the basepoint of . Now, to specify a pointed map from to , we need to give pointed maps from each of these pointed circles to . This is to say we need to associate to each element of a loop in (an element of the loop space ), and the trivial loop should be associated to the basepoint of : this is a pointed map from to . (The continuity of all involved maps needs to be checked.)

The adjunction is thus akin to currying, taking maps on cartesian products to their curried form, and is an example of Eckmann–Hilton duality.

This adjunction is a special case of the adjunction explained in the article on smash products.

## Desuspension

Desuspension is an operation partially inverse to suspension.[1]

## See also

## References

- Wolcott, Luke. "Imagining Negative-Dimensional Space" (PDF). forthelukeofmath.com. Retrieved 2015-06-23.

- Allen Hatcher,
*Algebraic topology.*Cambridge University Presses, Cambridge, 2002. xii+544 pp. ISBN 0-521-79160-X and ISBN 0-521-79540-0 *This article incorporates material from Suspension on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.*