reduction of structure group
Given a fiber bundle^{} $p:E\to B$ with typical fiber $F$ and structure group $G$ (henceforth called an $(F,G)$bundle over $B$), we say that the bundle admits a reduction^{} of its structure group to $H$, where $$ is a subgroup^{}, if it is isomorphic^{} to an $(F,H)$bundle over $B.$
Equivalently, $E$ admits a reduction of structure group to $H$ if there is a choice of local trivializations covering $E$ such that the transition functions^{} all belong to $H.$
Remark 1
Here, the action of $H$ on $F$ is the restriction^{} of the $G$action; in particular, this means that an $(F,H)$bundle is automatically an $(F,G)$bundle. The bundle isomorphism^{} in the definition then becomes meaningful in the category of $(F,G)$bundles over $B$.
Example 1
Let $H$ be the trivial subgroup. Then, the existence of a reduction of structure group to $H$ is equivalent^{} to the bundle being trivial.
For the following examples, let $E$ be an $n$dimensional vector bundle^{}, so that $F\cong {\mathbb{R}}^{n}$ with $G=GL(n,\mathbb{R}),$ the general linear group^{} acting as usual.
Example 2
Set $H=G{L}^{+}(n,\mathbb{R}),$ the subgroup of $GL(n,\mathbb{R})$ consisting of matrices with positive determinant^{}. A reduction to $H$ is equivalent to an orientation of the vector bundle. In the case where $B$ is a smooth manifold and $E=TB$ is its tangent bundle, this coincides with other definitions of an orientation of $B$.
Example 3
Set $H=O(n)$, the orthogonal group^{}. A reduction to $H$ is called a Riemannian or Euclidean structure on the vector bundle. It coincides with a continuous^{} fiberwise choice of a positive definite^{} inner product, and for the case of the tangent bundle, with the usual notion of a Riemannian metric on a manifold.
When $B$ is paracompact, an argument with partitions of unity^{} shows that a Riemannian structure always exists on any given vector bundle. For this reason, it is often convenient to start out assuming the structure group to be $O(n).$
Example 4
Let $n=2m$ be even, and let $H=GL(m,\u2102),$ the group of invertible^{} complex matrices, embedded in $GL(n,\mathbb{R})$ by means of the usual identification of $\u2102$ with ${\mathbb{R}}^{2}.$ A reduction to $H$ is called a complex structure on the vector bundle, and it is equivalent to a continuous fiberwise choice of an endomorphism $J$ satisfying ${J}^{2}=I.$
A complex structure on a tangent bundle is called an almostcomplex structure on the manifold. This is to distinguish it from the more restrictive notion of a complex structure on a manifold, which requires the existence of an atlas with charts in ${\u2102}^{m}$ such that the transition functions are holomorphic.
Example 5
Let $H=GL(1,\mathbb{R})\times GL(n1,\mathbb{R}),$ embedded in $GL(n,\mathbb{R})$ by $(A,B)\mapsto A\oplus B.$ A reduction to $H$ is equivalent to the existence of a splitting $E\cong {E}_{1}\oplus {E}_{2},$ where ${E}_{1}$ is a line bundle. More generally, a reduction to $GL(k,\mathbb{R})\times GL(nk,\mathbb{R})$ is equivalent to a splitting $E\cong {E}_{1}\oplus {E}_{2},$ where ${E}_{1}$ is a $k$plane bundle.
Remark 2
These examples all have two features in common, namely:
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a reduction to $H$ is equivalent to a continuous fiberbyfiber choice of a structure of the same kind.
For example, $O(n)$ is the subgroup of $GL(n,\mathbb{R})$ which preserves the standard inner product of ${\mathbb{R}}^{n},$ and reduction of structure to $O(n)$ is equivalent to a fiberwise choice of inner products.
This is not a coincidence. The intuition behind this is as follows. There is no obstacle to choosing a fiberwise inner product in a neighborhood of any given point $x\in B$: we simply choose a neighborhood $U$ on which the bundle is trivial, and with respect to a trivialization ${p}^{1}(U)\cong {\mathbb{R}}^{n}\times U$, we can let the inner product on each ${p}^{1}(y)$ be the standard inner product. However, if we make these choices locally around every point in $B$, there is no guarantee that they “glue together” properly to yield a global continuous choice, unless the transition functions preserve the standard inner product. But this is precisely what reduction of structure to $O(n)$ means.
The same explanation holds for subgroups preserving other kinds of structure.
Title  reduction of structure group 
Canonical name  ReductionOfStructureGroup 
Date of creation  20130322 13:26:06 
Last modified on  20130322 13:26:06 
Owner  antonio (1116) 
Last modified by  antonio (1116) 
Numerical id  12 
Author  antonio (1116) 
Entry type  Definition 
Classification  msc 55R10 
Related topic  VectorBundle 
Related topic  FiberBundle 
Defines  Euclidean structure 
Defines  Riemannian structure 
Defines  complex structure 
Defines  almostcomplex structure 