Steenrod problem
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In mathematics, and particularly homology theory, Steenrod's Problem (named after mathematician Norman Steenrod) is a problem concerning the realisation of homology classes by singular manifolds.[1]
Formulation
Let be a closed, oriented manifold of dimension , and let be its orientation class. Here denotes the integral, -dimensional homology group of . Any continuous map defines an induced homomorphism .[2] A homology class of is called realisable if it is of the form where . The Steenrod problem is concerned with describing the realisable homology classes of .[3]
Results
All elements of are realisable by smooth manifolds provided . Moreover, any cycle can be realized by the mapping of a pseudo-manifold.[3]
The assumption that M be orientable can be relaxed. In the case of non-orientable manifolds, every homology class of , where denotes the integers modulo 2, can be realized by a non-oriented manifold, .[3]
Conclusions
For smooth manifolds M the problem reduces to finding the form of the homomorphism , where is the oriented bordism group of X.[4] The connection between the bordism groups and the Thom spaces MSO(k) clarified the Steenrod problem by reducing it to the study of the homomorphisms .[3][5] In his landmark paper from 1954,[5] René Thom produced an example of a non-realisable class, , where M is the Eilenberg–MacLane space .
See also
References
External links
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