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Fibered manifold

From Wikipedia, the free encyclopedia
Concept in differential geometry

Indifferential geometry, in the category ofdifferentiable manifolds, afibered manifold is asurjectivesubmersionπ:EB{\displaystyle \pi :E\to B\,}that is, a surjective differentiable mapping such that at each pointyE{\displaystyle y\in E} the tangent mappingTyπ:TyETπ(y)B{\displaystyle T_{y}\pi :T_{y}E\to T_{\pi (y)}B}is surjective, or, equivalently, its rank equalsdimB.{\displaystyle \dim B.}[1]

History

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Intopology, the wordsfiber (Faser in German) andfiber space (gefaserter Raum) appeared for the first time in a paper byHerbert Seifert in 1932, but his definitions are limited to a very special case.[2] The main difference from the present day conception of a fiber space, however, was that for Seifert what is now called thebase space (topological space) of a fiber (topological) spaceE{\displaystyle E} was not part of the structure, but derived from it as a quotient space ofE.{\displaystyle E.} The first definition offiber space is given byHassler Whitney in 1935 under the namesphere space, but in 1940 Whitney changed the name tosphere bundle.[3][4]

The theory of fibered spaces, of whichvector bundles,principal bundles, topologicalfibrations and fibered manifolds are a special case, is attributed toSeifert,Hopf,Feldbau,Whitney,Steenrod,Ehresmann,Serre, and others.[5][6][7][8][9]

Formal definition

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A triple(E,π,B){\displaystyle (E,\pi ,B)} whereE{\displaystyle E} andB{\displaystyle B} are differentiable manifolds andπ:EB{\displaystyle \pi :E\to B} is a surjective submersion, is called afibered manifold.[10]E{\displaystyle E} is called thetotal space,B{\displaystyle B} is called thebase.

Examples

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  • Every differentiablefiber bundle is afibered manifold.
  • Every differentiablecovering space is afibered manifold with discrete fiber.
  • In general, a fibered manifold need not be a fiber bundle: different fibers may have different topologies. An example of this phenomenon may be constructed by restricting projection to total space of any vector bundle over smooth manifold with removed base space embedded by global section, finitely many points or any closed submanifold not containing a fiber in general.

Properties

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Fibered coordinates

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LetB{\displaystyle B} (resp.E{\displaystyle E}) be ann{\displaystyle n}-dimensional (resp.p{\displaystyle p}-dimensional) manifold. A fibered manifold(E,π,B){\displaystyle (E,\pi ,B)} admitsfiber charts. We say that achart(V,ψ){\displaystyle (V,\psi )} onE{\displaystyle E} is afiber chart, or isadapted to the surjective submersionπ:EB{\displaystyle \pi :E\to B} if there exists a chart(U,φ){\displaystyle (U,\varphi )} onB{\displaystyle B} such thatU=π(V){\displaystyle U=\pi (V)} andu1=x1π,u2=x2π,,un=xnπ,{\displaystyle u^{1}=x^{1}\circ \pi ,\,u^{2}=x^{2}\circ \pi ,\,\dots ,\,u^{n}=x^{n}\circ \pi \,,}whereψ=(u1,,un,y1,,ypn).y0V,φ=(x1,,xn),π(y0)U.{\displaystyle {\begin{aligned}\psi &=\left(u^{1},\dots ,u^{n},y^{1},\dots ,y^{p-n}\right).\quad y_{0}\in V,\\\varphi &=\left(x^{1},\dots ,x^{n}\right),\quad \pi \left(y_{0}\right)\in U.\end{aligned}}}

The above fiber chart condition may be equivalently expressed byφπ=pr1ψ,{\displaystyle \varphi \circ \pi =\mathrm {pr} _{1}\circ \psi ,}wherepr1:Rn×RpnRn{\displaystyle {\mathrm {pr} _{1}}:{\mathbb {R} ^{n}}\times {\mathbb {R} ^{p-n}}\to {\mathbb {R} ^{n}}\,}is the projection onto the firstn{\displaystyle n} coordinates. The chart(U,φ){\displaystyle (U,\varphi )} is then obviously unique. In view of the above property, thefibered coordinates of a fiber chart(V,ψ){\displaystyle (V,\psi )} are usually denoted byψ=(xi,yσ){\displaystyle \psi =\left(x^{i},y^{\sigma }\right)} wherei{1,,n},{\displaystyle i\in \{1,\ldots ,n\},}σ{1,,m},{\displaystyle \sigma \in \{1,\ldots ,m\},}m=pn{\displaystyle m=p-n} the coordinates of the corresponding chart(U,φ){\displaystyle (U,\varphi )} onB{\displaystyle B} are then denoted, with the obvious convention, byφ=(xi){\displaystyle \varphi =\left(x_{i}\right)} wherei{1,,n}.{\displaystyle i\in \{1,\ldots ,n\}.}

Conversely, if a surjectionπ:EB{\displaystyle \pi :E\to B} admits afiberedatlas, thenπ:EB{\displaystyle \pi :E\to B} is a fibered manifold.

Local trivialization and fiber bundles

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LetEB{\displaystyle E\to B} be a fibered manifold andV{\displaystyle V} any manifold. Then an open covering{Uα}{\displaystyle \left\{U_{\alpha }\right\}} ofB{\displaystyle B} together with mapsψ:π1(Uα)Uα×V,{\displaystyle \psi :\pi ^{-1}\left(U_{\alpha }\right)\to U_{\alpha }\times V,}calledtrivialization maps, such thatpr1ψα=π, for all α{\displaystyle \mathrm {pr} _{1}\circ \psi _{\alpha }=\pi ,{\text{ for all }}\alpha }is alocal trivialization with respect toV.{\displaystyle V.}[13]

A fibered manifold together with a manifoldV{\displaystyle V} is afiber bundle withtypical fiber (or justfiber)V{\displaystyle V} if it admits a local trivialization with respect toV.{\displaystyle V.} The atlasΨ={(Uα,ψα)}{\displaystyle \Psi =\left\{\left(U_{\alpha },\psi _{\alpha }\right)\right\}} is then called abundle atlas.

See also

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Notes

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  1. ^Kolář, Michor & Slovák 1993, p. 11
  2. ^Seifert 1932
  3. ^Whitney 1935
  4. ^Whitney 1940
  5. ^Feldbau 1939
  6. ^Ehresmann 1947a
  7. ^Ehresmann 1947b
  8. ^Ehresmann 1955
  9. ^Serre 1951
  10. ^Krupka & Janyška 1990, p. 47
  11. ^Giachetta, Mangiarotti & Sardanashvily 1997, p. 11
  12. ^Giachetta, Mangiarotti & Sardanashvily 1997, p. 15
  13. ^Giachetta, Mangiarotti & Sardanashvily 1997, p. 13

References

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Historical

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External links

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