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Cohomology with compact support : ウィキペディア英語版
Cohomology with compact support
In mathematics, cohomology with compact support refers to certain cohomology theories, usually with some condition requiring that cocycles should have compact support.
==de Rham cohomology with compact support for smooth manifolds==
Given a manifold ''X'', let \Omega^k_(X) be the real vector space of ''k''-forms on ''X'' with compact support, and ''d'' be the standard exterior derivative. Then the de Rham cohomology groups with compact support H^q_(X) are the homology of the chain complex (\Omega^\bullet_(X),d):
:0 \to \Omega^0_(X) \to \Omega^1_(X) \to \Omega^2_(X) \to \cdots
''i.e.'', H^q_(X) is the vector space of closed ''q''-forms modulo that of exact ''q''-forms.
Despite their definition as the homology of an ascending complex, the de Rham groups with compact support demonstrate covariant behavior; for example, given the inclusion mapping ''j'' for an open set ''U'' of ''X'', extension of forms on ''U'' to ''X'' (by defining them to be 0 on ''X''–''U'') is a map j_
*: \Omega^\bullet_(U) \to \Omega^\bullet_(X) inducing a map
:j_
*: H^q_(U) \to H^q_(X).
They also demonstrate contravariant behavior with respect to proper maps - that is, maps such that the inverse image of every compact set is compact. Let ''f'': ''Y'' → ''X'' be such a map; then the pullback
:f^
*:
\Omega^q_(X) \to \Omega^q_(Y)
\sum_I g_I \, dx_ \wedge \ldots \wedge dx_ \mapsto
\sum_I(g_I \circ f) \, d(x_ \circ f) \wedge \ldots \wedge d(x_ \circ f)
induces a map
:H^q_(X) \to H^q_(Y).
If ''Z'' is a submanifold of ''X'' and ''U'' = ''X''–''Z'' is the complementary open set, there is a long exact sequence
:\cdots \to H^q_(U) \overset H^q_(X) \overset H^q_(Z) \overset H^_(U) \to \cdots
called the long exact sequence of cohomology with compact support. It has numerous applications, such as the Jordan curve theorem, which is obtained for ''X'' = R² and ''Z'' a simple closed curve in ''X''.
De Rham cohomology with compact support satisfies a covariant Mayer–Vietoris sequence: if ''U'' and ''V'' are open sets covering ''X'', then
:\cdots \to H^q_(U \cap V) \to H^q_(U)\oplus H^q_(V) \to H^q_(X) \overset H^_(U\cap V) \to \cdots
where all maps are induced by extension by zero is also exact.

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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