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Euler class : ウィキペディア英語版
Euler class
In mathematics, specifically in algebraic topology, the Euler class, named after Leonhard Euler, 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.
Throughout this article ''E'' → ''X'' is an oriented, real vector bundle of rank ''r''.
== Formal definition ==
The Euler class ''e''(''E'') is an element of the integral cohomology group
:H^r(X; \mathbf),
constructed as follows. An orientation of ''E'' amounts to a continuous choice of generator of the cohomology
:H^r(F, F \setminus F_0; \mathbf)
of each fiber ''F'' relative to the complement ''F''\''F''0 of its zero element ''F''0. From Thom isomorphism, this induces an orientation class
:u \in H^r(E, E \setminus E_0; \mathbf)
in the cohomology of ''E'' relative to the complement ''E''\''E''0 of the zero section ''E''0. The inclusions
:(X, \emptyset) \hookrightarrow (E, \emptyset) \hookrightarrow (E, E \setminus E_0),
where ''X'' includes into ''E'' as the zero section, induce maps
:H^r(E, E \setminus E_0; \mathbf) \to H^r(E; \mathbf) \to H^r(X; \mathbf).
The Euler class ''e''(''E'') is the image of ''u'' under the composition of these maps.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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