Thursday, 26 February 2009

at.algebraic topology - Explanation for the Thom-Pontryagin construction (and its generalisations)

In 1950, Pontryagin showed that the n-th framed cobordism group of smooth manifolds was equal to n-th stable homotopy group of spheres:



limktoinftypin+k(Sk)congOmegatextframedn.



Later on, in his 1954 paper, Thom generalises this with the now called Thom spaces, and shows that there is a similar correspondence for more general types of cobordism: manifolds with a (B,f) structure on their normal bundle; for example, unoriented cobordism for B=BO, oriented cobordism for B=BSO, complex cobordism for B=BU, framed cobordism for B=BI for the identity I in O, etc. (Thom considers the cases BO and BSO.)
The generalisation he arrived to, now called the Thom-Pontryagin construction, is the following:



limktoinftypin+k(TBk)congOmega(B,f)n,



where TB is the Thom space of the universal bundle over B given by the classifying map BtoBO; TB is obtained by adding a point at infinity to each fiber and gluing all these added points to a single point in the total space of the bundle.



In fact, the result can be generalised further by considering cobordism as a homology theory, and one arrives at the following:



Omega(B,f)n(X,Y)conglimktoinftypin+k(X/YwedgeTBk),



where, if Y is empty, X/Y is the disjoint union of X with a point (and wedge is the smash product). Here Omegan(X,emptyset) is to be understood as a relative cobordism over X. This clearly generalises the previous result by taking X to be a point (and Y empty).



Now, my question is, how do you understand the Thom-Pontryagin construction? I've seen a few mentions of a particularly visual way of understanding it, but without much to actually back this up (besides from, I remember, a few mentions of blobs of ink). The standard proofs (in Stong's Notes on Cobordism Theory or in Thom's original paper for example) are quite long and I have trouble keeping hold of my geometric intuition throughout.

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