Integration along fibers
# 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". It is also called the fiber integration.

## Definition

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 for tangent vectors $w_i's$ at b, let

    $(\pi _{*}\alpha )_{b}(w_{1},\dots ,w_{k-m})=\int _{\pi ^{-1}(b)}\beta$

where $\beta$ is the induced top-form on the fiber $\pi ^{-1}(b)$; i.e., an $m$-form given by: with ${\widetilde {w_{i}}}$ lifts of $w_{i}$ to $E$,

    $\beta (v_{1},\dots ,v_{m})=\alpha (v_{1},\dots ,v_{m},{\widetilde {w_{1}}},\dots ,{\widetilde {w_{k-m}}}).$

(To see $b\mapsto (\pi _{*}\alpha )_{b}$ is smooth, work it out in coordinates; cf. an example below.)

Then $\pi _{*}$ is a linear map $\Omega ^{k}(E)\to \Omega ^{k-m}(B)$. By Stokes' formula, if the fibers have no boundaries(i.e. $[d,\int ]=0$), the map descends to de Rham cohomology:

    $\pi _{*}:\operatorname {H} ^{k}(E;\mathbb {R} )\to \operatorname {H} ^{k-m}(B;\mathbb {R} ).$

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 $0\to K\to \Omega ^{*}(E){\overset {\pi _{*}}{\to }}\Omega ^{*}(B)\to 0$, K the kernel, which leads to a long exact sequence, dropping the coefficient $\mathbb {R}$ and using $\operatorname {H} ^{k}(B)\simeq \operatorname {H} ^{k+m}(K)$:

    $\cdots \rightarrow \operatorname {H} ^{k}(B){\overset {\delta }{\to }}\operatorname {H} ^{k+m+1}(B){\overset {\pi ^{*}}{\rightarrow }}\operatorname {H} ^{k+m+1}(E){\overset {\pi _{*}}{\rightarrow }}\operatorname {H} ^{k+1}(B)\rightarrow \cdots$,

called the Gysin sequence.

## Example

Let $\pi :M\times [0,1]\to M$ be an obvious projection. First assume $M=\mathbb {R} ^{n}$ with coordinates $x_{j}$ and consider a k-form:

    $\alpha =f\,dx_{i_{1}}\wedge \dots \wedge dx_{i_{k}}+g\,dt\wedge dx_{j_{1}}\wedge \dots \wedge dx_{j_{k-1}}.$

Then, at each point in M,

    $\pi _{*}(\alpha )=\pi _{*}(g\,dt\wedge dx_{j_{1}}\wedge \dots \wedge dx_{j_{k-1}})=\left(\int _{0}^{1}g(\cdot ,t)\,dt\right)\,{dx_{j_{1}}\wedge \dots \wedge dx_{j_{k-1}}}.$

From this local calculation, the next formula follows easily (see Poincaré_lemma#Direct_proof): if $\alpha$ is any k-form on $M\times [0,1],$

    $\pi _{*}(d\alpha )=\alpha _{1}-\alpha _{0}-d\pi _{*}(\alpha )$

where $\alpha _{i}$ is the restriction of $\alpha$ to $M\times \{i\}$.

As an application of this formula, let $f:M\times [0,1]\to N$ be a smooth map (thought of as a homotopy). Then the composition $h=\pi _{*}\circ f^{*}$ is a homotopy operator (also called a chain homotopy):

    $d\circ h+h\circ d=f_{1}^{*}-f_{0}^{*}:\Omega ^{k}(N)\to \Omega ^{k}(M),$

which implies $f_{1},f_{0}$ induce the same map on cohomology, the fact known as the homotopy invariance of de Rham cohomology. As a corollary, for example, let U be an open ball in $R^n$ with center at the origin and let $f_{t}:U\to U,x\mapsto tx$. Then $\operatorname {H} ^{k}(U;\mathbb {R} )=\operatorname {H} ^{k}(pt;\mathbb {R} )$, the fact known as the Poincaré lemma.

## Projection formula

Given a vector bundle π : E → B over a manifold, we say a differential form α on E has vertical-compact support if the restriction $\alpha |_{\pi ^{-1}(b)}$ has compact support for each b in B. We write $\Omega _{vc}^{*}(E)$ for the vector space of differential forms on E with vertical-compact support. If E is oriented as a vector bundle, exactly as before, we can define the integration along the fiber:

    $\pi _{*}:\Omega _{vc}^{*}(E)\to \Omega ^{*}(B).$

The following is known as the projection formula. We make $\Omega _{vc}^{*}(E)$ a right $\Omega ^{*}(B)$-module by setting $\alpha \cdot \beta =\alpha \wedge \pi ^{*}\beta$.

Proposition — Let $\pi :E\to B$ be an oriented vector bundle over a manifold and $\pi _{*}$ the integration along the fiber. Then

* $\pi _{*}$ is $\Omega ^{*}(B)$-linear; i.e., for any form β on B and any form α on E with vertical-compact support,

    $\pi _{*}(\alpha \wedge \pi ^{*}\beta )=\pi _{*}\alpha \wedge \beta .$
* If B is oriented as a manifold, then for any form α on E with vertical compact support and any form β on B with compact support,

    $\int _{E}\alpha \wedge \pi ^{*}\beta =\int _{B}\pi _{*}\alpha \wedge \beta$.

Proof: 1. Since the assertion is local, we can assume π is trivial: i.e., $\pi :E=B\times \mathbb {R} ^{n}\to B$ is a projection. Let $t_{j}$ be the coordinates on the fiber. If $\alpha =g\,dt_{1}\wedge \cdots \wedge dt_{n}\wedge \pi ^{*}\eta$, then, since $\pi ^{*}$ is a ring homomorphism,

    $\pi _{*}(\alpha \wedge \pi ^{*}\beta )=\left(\int _{\mathbb {R} ^{n}}g(\cdot ,t_{1},\dots ,t_{n})dt_{1}\dots dt_{n}\right)\eta \wedge \beta =\pi _{*}(\alpha )\wedge \beta .$

Similarly, both sides are zero if α does not contain dt. The proof of 2. is similar. $\square$

