In mathematics, the **Thom space,** **Thom complex,** or **Pontryagin–Thom construction** (named after René Thom and Lev Pontryagin) of algebraic topology and differential topology is a topological space associated to a vector bundle, over any paracompact space.

## Construction of the Thom space

One way to construct this space is as follows. Let

be a rank *n* real vector bundle over the paracompact space *B*. Then for each point *b* in *B*, the fiber is an -dimensional real vector space. Choose an orthogonal structure on E, a smoothly varying inner product on the fibers; we can do this using partitions of unity. Let be the unit disk bundle with respect to our orthogonal structure, and let be the unit sphere bundle, then the **Thom space** is the quotient of topological spaces. is a pointed space with the image of in the quotient as basepoint. If *B* is compact, then is the one-point compactification of *E*.

For example, if *E* is the trivial bundle , then and . Writing for *B* with a disjoint basepoint, is the smash product of and ; that is, the *n*-th reduced suspension of .

## The Thom isomorphism

The significance of this construction begins with the following result, which belongs to the subject of cohomology of fiber bundles. (We have stated the result in terms of coefficients to avoid complications arising from orientability; see also Orientation of a vector bundle#Thom space.)

Let be a real vector bundle of rank *n*. Then there is an isomorphism, now called a **Thom isomorphism**

for all *k* greater than or equal to 0, where the right hand side is reduced cohomology.

This theorem was formulated and proved by René Thom in his famous 1952 thesis.

We can interpret the theorem as a global generalization of the suspension isomorphism on local trivializations, because the Thom space of a trivial bundle on *B* of rank *k* is isomorphic to the *k*th suspension of , *B* with a disjoint point added (cf. #Construction of the Thom space.) This can be more easily seen in the formulation of the theorem that does not make reference to Thom space:

**Thom isomorphism** —
Let be a ring and be an oriented real vector bundle of rank *n*. Then there exists a class

where *B* is embedded into *E* as a zero section, such that for any fiber *F* the restriction of *u*

is the class induced by the orientation of *F*. Moreover,

is an isomorphism.

In concise terms, the last part of the theorem says that *u* freely generates as a right -module. The class *u* is usually called the **Thom class** of *E*. Since the pullback is a ring isomorphism, is given by the equation:

In particular, the Thom isomorphism sends the identity element of to *u*. Note: for this formula to make sense, *u* is treated as an element of (we drop the ring )

^{[1]}

## Significance of Thom's work

In his 1952 paper, Thom showed that the Thom class, the Stiefel–Whitney classes, and the Steenrod operations were all related. He used these ideas to prove in the 1954 paper *Quelques propriétés globales des variétés differentiables* that the cobordism groups could be computed as the homotopy groups of certain Thom spaces *MG*(*n*). The proof depends on and is intimately related to the transversality properties of smooth manifolds—see Thom transversality theorem. By reversing this construction, John Milnor and Sergei Novikov (among many others) were able to answer questions about the existence and uniqueness of high-dimensional manifolds: this is now known as surgery theory. In addition, the spaces *MG(n)* fit together to form spectra *MG* now known as **Thom spectra**, and the cobordism groups are in fact stable. Thom's construction thus also unifies differential topology and stable homotopy theory, and is in particular integral to our knowledge of the stable homotopy groups of spheres.

If the Steenrod operations are available, we can use them and the isomorphism of the theorem to construct the Stiefel–Whitney classes. Recall that the Steenrod operations (mod 2) are natural transformations

defined for all nonnegative integers *m*. If , then coincides with the cup square. We can define the *i*th Stiefel–Whitney class of the vector bundle by:

## Consequences for differentiable manifolds

If we take the bundle in the above to be the tangent bundle of a smooth manifold, the conclusion of the above is called the Wu formula, and has the following strong consequence: since the Steenrod operations are invariant under homotopy equivalence, we conclude that the Stiefel–Whitney classes of a manifold are as well. This is an extraordinary result that does not generalize to other characteristic classes. There exists a similar famous and difficult result establishing topological invariance for rational Pontryagin classes, due to Sergei Novikov.

## Thom spectrum

### Real cobordism

There are two ways to think about bordism: one as considering two -manifolds are cobordant if there is an -manifold with boundary such that

Another technique to encode this kind of information is to take an embedding and considering the normal bundle

The embedded manifold together with the isomorphism class of the normal bundle actually encodes the same information as the cobordism class . This can be shown^{[2]} by using a cobordism and finding an embedding to some which gives a homotopy class of maps to the Thom space defined below. Showing the isomorphism of

requires a little more work.^{[3]}

### Definition of Thom spectrum

By definition, the **Thom spectrum**^{[4]} is a sequence of Thom spaces

where we wrote for the universal vector bundle of rank *n*. The sequence forms a spectrum.^{[5]} A theorem of Thom says that is the unoriented cobordism ring;^{[6]} the proof of this theorem relies crucially on Thom’s transversality theorem.^{[7]} The lack of transversality prevents from computing cobordism rings of, say, topological manifolds from Thom spectra.

## See also

## Notes

**^**Proof of the isomorphism. We can embed*B*into either as the zero section; i.e., a section at zero vector or as the infinity section; i.e., a section at infinity vector (topologically the difference is immaterial.) Using two ways of embedding we have the triple:- .

*B*. Taking the long exact sequence of this triple, we then see:**^**"Thom's theorem" (PDF). Archived (PDF) from the original on 17 Jan 2021.**^**"Transversality" (PDF). Archived (PDF) from the original on 17 Jan 2021.**^**See pp. 8-9 in Greenlees, J. P. C. (2006-09-15). "Spectra for commutative algebraists". arXiv:math/0609452.**^**http://math.northwestern.edu/~jnkf/classes/mflds/2cobordism.pdf**^**Stong, p. 18**^**http://math.northwestern.edu/~jnkf/classes/mflds/4transversality.pdf

## References

- Sullivan, Dennis (2004). "René Thom's Work on Geometric Homology and Bordism".
*Bulletin of the American Mathematical Society*.**41**(3): 341–350. doi:10.1090/S0273-0979-04-01026-2. - Bott, Raoul; Tu, Loring (1982).
*Differential Forms in Algebraic Topology*. New York: Springer. ISBN 0-387-90613-4. A classic reference for differential topology, treating the link to Poincaré duality and the Euler class of Sphere bundles - May, J. Peter (1999).
*A Concise Course in Algebraic Topology*. University of Chicago Press. pp. 183–198. ISBN 0-226-51182-0. - "Explanation for the Pontryagin–Thom construction".
*MathOverflow*. - Stong, Robert E. (1968).
*Notes on cobordism theory*. Princeton University Press. - Thom, René (1954). "Quelques propriétés globales des variétés différentiables".
*Commentarii Mathematici Helvetici*.**28**: 17–86. doi:10.1007/BF02566923. S2CID 120243638. - Ando, Matthew; Blumberg, Andrew J.; Gepner, David J.; Hopkins, Michael J.; Rezk, Charles (2014). "Units of ring spectra and Thom spectra".
*Journal of Topology*.**7**(4): 1077–1117. arXiv:0810.4535. doi:10.1112/jtopol/jtu009. MR 0286898. S2CID 119613530.

## External links

- http://ncatlab.org/nlab/show/Thom+spectrum
- "Thom space",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - Akhil Mathew's blog posts: https://amathew.wordpress.com/tag/thom-space/