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单词 SectionOfAFiberBundle
释义

section of a fiber bundle


Let p:EB be a fiber bundleMathworldPlanetmath, denoted by ξ.

A sectionPlanetmathPlanetmathPlanetmathPlanetmath of ξis a continuous mapMathworldPlanetmath s:BE such that the composition ps equals the identity.That is, for every bB, s(b) is an element of the fiber over b.More generally, given a topological subspace A of B, a section of ξ over A is a section of the restricted bundlep|A:p-1(A)A.

The set of sections of ξ over A is often denoted by Γ(A;ξ), orby Γ(ξ) for sections defined on all of B. Elements of Γ(ξ) are sometimescalled global sections, in contrast with the local sections Γ(U;ξ) defined on an open set U.

Remark 1

If E and B have, for example, smooth structuresMathworldPlanetmath, one can talk about smoothsections of the bundle. According to the context, the notation Γ(ξ) oftendenotes smooth sections, or some other set of suitably restricted sections.

Example 1

If ξ is a trivial fiber bundle with fiber F, so that E=F×B andp is projectionPlanetmathPlanetmath to B, then sections of ξ are in a natural bijectiveMathworldPlanetmath correspondence with continuous functionsMathworldPlanetmath BF.

Example 2

If B is a smooth manifold and E=TB its tangent bundle, a (smooth) section of this bundle is precisely a (smooth) tangent vectorMathworldPlanetmath field.

In fact, any tensor field on a smooth manifold M is a section ofan appropriate vector bundle. For instance, a contravariant k-tensor field is a section of the bundle TMk obtained by repeated tensor product from the tangent bundle, and similarly for covariant and mixed tensor fields.

Example 3

If B is a smooth manifold which is smoothly embedded in a Riemannian manifoldMathworldPlanetmathM, we can let the fiber over bB be the orthogonal complementMathworldPlanetmathPlanetmath in TbM of the tangent spaceMathworldPlanetmath TbB of B at b. These choices of fiber turn out tomake up a vector bundle ν(B) over B, called the of B. A section of ν(B) is a normalvectorMathworldPlanetmath field on B.

Example 4

If ξ is a vector bundle, the zero section is defined simply bys(b)=0, the zero vectorMathworldPlanetmath on the fiber.

It is interesting to ask if a vector bundle admits a section which isnowhere zero. The answer is yes, for example, in the case of a trivial vectorbundle, but in general it depends on the topologyMathworldPlanetmath of the spaces involved.A well-known case of this question is the hairy ball theoremMathworldPlanetmath, whichsays that there are no nonvanishing tangent vector fields on the sphere.

Example 5

If ξ is a principal (http://planetmath.org/PrincipalBundle) G-bundle (http://planetmath.org/PrincipalBundle), the existence of any section isequivalentMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath to the bundle being trivial.

Remark 2

The correspondence taking an open set U in B to Γ(U;ξ) is an exampleof a sheaf on B.

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更新时间:2025/5/4 8:28:03