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In [[mathematics]], a '''pullback bundle''' or '''induced bundle''' <ref>{{cite book | last = Steenrod | first = Norman | title = The Topology of Fibre Bundles | publisher = Princeton University Press | location = Princeton | year = 1951 | isbn = 0-691-00548-6}} page 47 | |||
</ref> <ref>{{cite book | last = Husemoller | first = Dale | title = Fibre Bundles | publisher = Springer | edition = Third |location = New York | year=1994 | isbn=978-0-387-94087-8}} page 18 | |||
</ref> <ref>{{Cite book | last1=Lawson | first1=H. Blaine | last2=Michelsohn |author2-link=Marie-Louise Michelsohn| first2=Marie-Louise | title=Spin Geometry | publisher=[[Princeton University Press]] | isbn=978-0-691-08542-5 | year=1989 | postscript=<!--None-->}} page 374 </ref> is a useful construction in the theory of [[fiber bundle]]s. Given a fiber bundle ''π'' : ''E'' → ''B'' and a [[continuous (topology)|continuous map]] ''f'' : ''B''′ → ''B'' one can define a "pullback" of ''E'' by ''f'' as a bundle ''f'' *''E'' over ''B''′. The fiber of ''f'' *''E'' over a point '' b' '' in ''B''′ is just the fiber of ''E'' over ''f''('' b' ''). Thus ''f'' *''E'' is the [[disjoint union]] of all these fibers equipped with a suitable [[topological space|topology]]. | |||
==Formal definition== | |||
Let ''π'' : ''E'' → ''B'' be a fiber bundle with abstract fiber ''F'' and let ''f'' : ''B''′ → ''B'' be a [[continuous (topology)|continuous map]]. Define the '''pullback bundle''' by | |||
:<math>f^{*}E = \{(b',e) \in B' \times E \mid f(b') = \pi(e)\}\subset B'\times E </math> | |||
and equip it with the [[subspace topology]] and the [[projection map]] π′ : ''f''<sup>*</sup>''E'' → ''B''′ given by the projection onto the first factor, i.e., | |||
:<math>\pi'(b',e) = b'.\,</math> | |||
The projection onto the second factor gives a map <math>\tilde f \colon f^{*}E \to E</math> such that the following diagram [[commutative diagram|commutes]]: | |||
::<math>\begin{array} {ccc} | |||
f^{\ast}E & \stackrel {\tilde f} {\longrightarrow} & E\\ | |||
{\pi}' \downarrow & & \downarrow \pi\\ | |||
B' & \stackrel f {\longrightarrow} & B | |||
\end{array}</math> | |||
If (''U'', φ) is a [[local trivialization]] of ''E'' then (''f''<sup>−1</sup>''U'', ψ) is a local trivialization of ''f''<sup>*</sup>''E'' where | |||
:<math>\psi(b',e) = (b', \mbox{proj}_2(\varphi(e))).\,</math> | |||
It then follows that ''f''<sup>*</sup>''E'' is a fiber bundle over ''B''′ with fiber ''F''. The bundle ''f''<sup>*</sup>''E'' is called the '''pullback of ''E'' by ''f'' ''' or the '''bundle induced by ''f'''''. The map <math>\tilde f</math> is then a [[bundle morphism]] covering ''f''. | |||
==Properties== | |||
Any [[section (fiber bundle)|section]] ''s'' of ''E'' over ''B'' induces a section of ''f''<sup>*</sup>''E'', called the '''pullback section''' ''f''<sup>*</sup>''s'', simply by defining <math>f^*s=s\circ f</math>. | |||
If the bundle ''E'' → ''B'' has [[structure group]] ''G'' with transition functions ''t''<sub>''ij''</sub> (with respect to a family of local trivializations {(''U''<sub>''i''</sub>, φ<sub>''i''</sub>)} ) then the pullback bundle ''f''<sup>*</sup>''E'' also has structure group ''G''. The transition functions in ''f''<sup>*</sup>''E'' are given by | |||
:<math>f^{*}t_{ij} = t_{ij} \circ f.</math> | |||
If ''E'' → ''B'' is a [[vector bundle]] or [[principal bundle]] then so is the pullback ''f''<sup>*</sup>''E''. In the case of a principal bundle the right [[group action|action]] of ''G'' on ''f''<sup>*</sup>''E'' is given by | |||
:<math>(x,e)\cdot g = (x,e\cdot g)</math> | |||
It then follows that the map <math>\tilde f</math> is [[equivariant]] and so defines a morphism of principal bundles. | |||
In the language of [[category theory]], the pullback bundle construction is an example of the more general [[categorical pullback]]. As such it satisfies the corresponding [[universal property]]. | |||
The construction of the pullback bundle can be carried out in subcategories of the category of [[topological spaces]], such as the category of [[smooth manifold]]s. The latter construction is useful in [[differential geometry and topology]] | |||
Examples: It is illuminating to consider the pullback of the degree 2 map from the circle to itself over the degree 3 or 4 map from the circle to itself. In such examples one sometimes gets a connected and sometimes disconnected space, but always several copies of the circle. | |||
==Bundles and sheaves== | |||
Bundles may also be described by their [[sheaf (mathematics)|sheaves of sections]]. The pullback of bundles then corresponds to the [[Inverse image functor|inverse image of sheaves]], which is a [[Covariance and contravariance of functors|contravariant]] functor. A sheaf, however, is more naturally a [[Covariance and contravariance of functors|covariant]] object, since it has a [[pushforward]], called the [[direct image of a sheaf]]. The tension and interplay between bundles and sheaves, or inverse and direct image, can be advantageous in many areas of geometry. However, the direct image of a sheaf of sections of a bundle is ''not'' in general the sheaf of sections of some direct image bundle, so that although the notion of a 'pushforward of a bundle' is defined in some contexts (for example, the pushforward by a diffeomorphism), in general it is better understood in the category of sheaves, because the objects it creates cannot in general be bundles. | |||
==References== | |||
<references/> | |||
==Books== | |||
*{{cite book | last = Steenrod | first = Norman | title = The Topology of Fibre Bundles | publisher = Princeton University Press | location = Princeton | year = 1951 | isbn = 0-691-00548-6}} | |||
*{{cite book | last = Husemoller | first = Dale | title = Fibre Bundles | publisher = Springer | edition = Third |location = New York | year=1994 | isbn=978-0-387-94087-8}} | |||
*{{cite book | last = Sharpe | first = R. W. | title = Differential Geometry: Cartan's Generalization of Klein's Erlangen Program | publisher = Springer | location = New York | year=1997 | isbn=0-387-94732-9}} | |||
==External links== | |||
* [http://planetmath.org/encyclopedia/PullbackBundle.html Pullback Bundle], PlanetMath | |||
[[Category:Fiber bundles]] | |||
Revision as of 19:46, 15 January 2014
In mathematics, a pullback bundle or induced bundle [1] [2] [3] is a useful construction in the theory of fiber bundles. Given a fiber bundle π : E → B and a continuous map f : B′ → B one can define a "pullback" of E by f as a bundle f *E over B′. The fiber of f *E over a point b' in B′ is just the fiber of E over f( b' ). Thus f *E is the disjoint union of all these fibers equipped with a suitable topology.
Formal definition
Let π : E → B be a fiber bundle with abstract fiber F and let f : B′ → B be a continuous map. Define the pullback bundle by
and equip it with the subspace topology and the projection map π′ : f*E → B′ given by the projection onto the first factor, i.e.,
The projection onto the second factor gives a map such that the following diagram commutes:
If (U, φ) is a local trivialization of E then (f−1U, ψ) is a local trivialization of f*E where
It then follows that f*E is a fiber bundle over B′ with fiber F. The bundle f*E is called the pullback of E by f or the bundle induced by f. The map is then a bundle morphism covering f.
Properties
Any section s of E over B induces a section of f*E, called the pullback section f*s, simply by defining .
If the bundle E → B has structure group G with transition functions tij (with respect to a family of local trivializations {(Ui, φi)} ) then the pullback bundle f*E also has structure group G. The transition functions in f*E are given by
If E → B is a vector bundle or principal bundle then so is the pullback f*E. In the case of a principal bundle the right action of G on f*E is given by
It then follows that the map is equivariant and so defines a morphism of principal bundles.
In the language of category theory, the pullback bundle construction is an example of the more general categorical pullback. As such it satisfies the corresponding universal property.
The construction of the pullback bundle can be carried out in subcategories of the category of topological spaces, such as the category of smooth manifolds. The latter construction is useful in differential geometry and topology
Examples: It is illuminating to consider the pullback of the degree 2 map from the circle to itself over the degree 3 or 4 map from the circle to itself. In such examples one sometimes gets a connected and sometimes disconnected space, but always several copies of the circle.
Bundles and sheaves
Bundles may also be described by their sheaves of sections. The pullback of bundles then corresponds to the inverse image of sheaves, which is a contravariant functor. A sheaf, however, is more naturally a covariant object, since it has a pushforward, called the direct image of a sheaf. The tension and interplay between bundles and sheaves, or inverse and direct image, can be advantageous in many areas of geometry. However, the direct image of a sheaf of sections of a bundle is not in general the sheaf of sections of some direct image bundle, so that although the notion of a 'pushforward of a bundle' is defined in some contexts (for example, the pushforward by a diffeomorphism), in general it is better understood in the category of sheaves, because the objects it creates cannot in general be bundles.
References
- ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 page 47 - ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 page 18 - ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 page 374
Books
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 - 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 - 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
External links
- Pullback Bundle, PlanetMath