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Euler class

In mathematics, specifically in algebraic topology, the Euler class is a characteristic class of oriented, real vector bundles. Like other characteristic classes, it measures how "twisted" the vector bundle is. In the case of the tangent bundle of a smooth manifold, it generalizes the classical notion of Euler characteristic. It is named after Leonhard Euler because of this.

Throughout this article is an oriented, real vector bundle of rank over a base space .

Formal definition

The Euler class is an element of the integral cohomology group

constructed as follows. An orientation of amounts to a continuous choice of generator of the cohomology

of each fiber relative to the complement of zero. From the Thom isomorphism, this induces an orientation class

in the cohomology of relative to the complement of the zero section . The inclusions

where includes into as the zero section, induce maps

The Euler class e(E) is the image of u under the composition of these maps.

Properties

The Euler class satisfies these properties, which are axioms of a characteristic class:

  • Functoriality: If is another oriented, real vector bundle and is continuous and covered by an orientation-preserving map , then . In particular, .
  • Whitney sum formula: If is another oriented, real vector bundle, then the Euler class of their direct sum is given by
  • Normalization: If possesses a nowhere-zero section, then .
  • Orientation: If is with the opposite orientation, then .

Note that "Normalization" is a distinguishing feature of the Euler class. The Euler class obstructs the existence of a non-vanishing section in the sense that if then has no non-vanishing section.

Also unlike other characteristic classes, it is concentrated in a degree which depends on the rank of the bundle: . By contrast, the Stiefel Whitney classes live in independent of the rank of . This reflects the fact that the Euler class is unstable, as discussed below.

Vanishing locus of generic section

The Euler class corresponds to the vanishing locus of a section of in the following way. Suppose that is an oriented smooth manifold of dimension . Let be a smooth section that transversely intersects the zero section. Let be the zero locus of . Then is a codimension submanifold of which represents a homology class and is the Poincaré dual of .

Self-intersection

For example, if is a compact submanifold, then the Euler class of the normal bundle of in is naturally identified with the self-intersection of in .

Relations to other invariants

In the special case when the bundle E in question is the tangent bundle of a compact, oriented, r-dimensional manifold, the Euler class is an element of the top cohomology of the manifold, which is naturally identified with the integers by evaluating cohomology classes on the fundamental homology class. Under this identification, the Euler class of the tangent bundle equals the Euler characteristic of the manifold. In the language of characteristic numbers, the Euler characteristic is the characteristic number corresponding to the Euler class.

Thus the Euler class is a generalization of the Euler characteristic to vector bundles other than tangent bundles. In turn, the Euler class is the archetype for other characteristic classes of vector bundles, in that each "top" characteristic class equals the Euler class, as follows.

Modding out by 2 induces a map

The image of the Euler class under this map is the top Stiefel-Whitney class wr(E). One can view this Stiefel-Whitney class as "the Euler class, ignoring orientation".

Any complex vector bundle E of complex rank d can be regarded as an oriented, real vector bundle E of real rank 2d. The Euler class of E is given by the highest dimensional Chern class

Squares to top Pontryagin class

The Pontryagin class is defined as the Chern class of the complexification of E: .

The complexification is isomorphic as an oriented bundle to . Comparing Euler classes, we see that

If the rank r of E is even then where is the top dimensional Pontryagin class of .

Instability

A characteristic class is stable if where is a rank one trivial bundle. Unlike most other characteristic classes, the Euler class is unstable. In fact, .

The Euler class is represented by a cohomology class in the classifying space BSO(k) . The unstability of the Euler class shows that it is not the pull-back of a class in


Zdroj: Wikipedia.org - čítajte viac o Euler class





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