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Integration along fibers
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Integration along fibers : ウィキペディア英語版
Integration along fibers
In differential geometry, the integration along fibers of a ''k''-form yields a (k-m)-form where ''m'' is the dimension of the fiber, via "integration". More precisely, let \pi: E \to B be a fiber bundle over a manifold with compact oriented fibers. If \alpha is a ''k''-form on ''E'', then let:
: (\pi_
* \alpha)_b(w_1, \dots, w_) = \int_ \beta
where \beta is the induced top-form on the fiber \pi^(b); i.e., an m-form given by
:\beta(v_1, \dots, v_m) = \alpha(\widetilde, \dots, \widetilde\text w_i.
(To see b \mapsto (\pi_
* \alpha)_b is smooth, work it out in coordinates; cf. an example below.)
\pi_
* is then a linear map \Omega^k(E) \to \Omega^(B), which is in fact surjective. By Stokes' formula, if the fibers have no boundaries, the map descends to de Rham cohomology:
:\pi_
*: \operatorname^k(E) \to \operatorname^(B).
This is also called the fiber integration. Now, suppose \pi is a sphere bundle; i.e., the typical fiber is a sphere. Then there is an exact sequence 0 \to K \to \Omega^
*(E) \overset\to \Omega^
*(B) \to 0, ''K'' the kernel,
which leads to a long exact sequence, using \operatorname^k(B) \simeq \operatorname^(K):
:\dots \rightarrow \operatorname^k(B) \overset\to \operatorname^(B) \overset \rightarrow \operatorname^(E) \overset \rightarrow \operatorname^(B) \rightarrow \dots,
called the Gysin sequence.
== Example ==
Let \pi: M \times (1 ) \to M be an obvious projection. For simplicity, assume M = \mathbb^n with coordinates x_j and consider a ''k''-form:
:\alpha = f \, dx_ \wedge \dots \wedge dx_ + g \, dt \wedge dx_ \wedge \dots \wedge dx_ \wedge \dots \wedge dx_ \wedge \dots \wedge dx_.
From this the next formula follows easily: if \alpha is any ''k''-form on M \times I,
:\pi_
*(d \alpha) = \alpha_1 - \alpha_0 - d \pi_
*(\alpha)
where \alpha_i is the restriction of \alpha to M \times \. This formula is a special case of Stokes' formula. As an application of this, let f: M \times (1 ) \to N be a smooth map (thought of as a homotopy). Then the composition h = \pi_
* \circ f^
* is a homotopy operator:
:d \circ h + h \circ d = f_1^
* - f_0^
*: \Omega^k(N) \to \Omega^k(M),
which implies f_1, f_0 induces the same map on cohomology. For example, let ''U'' be an open ball with center at the origin and let f_t: U \to U, x \mapsto tx. Then \operatorname^k(U) = \operatorname^k(pt), the fact known as the Poincaré lemma.
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