Frame bundle explained

F(E)

associated with any vector bundle E

. The fiber of

F(E)

over a point x

is the set of all ordered bases, or frames, for E_x

. The general linear group acts naturally on

F(E)

via a change of basis, giving the frame bundle the structure of a principal GL(k,R)

-bundle (where k is the rank of E

).

The frame bundle of a smooth manifold is the one associated with its tangent bundle. For this reason it is sometimes called the tangent frame bundle.

Definition and construction

Let

E\toX

be a real vector bundle of rank

over a topological space

. A frame at a point

x\inX

is an ordered basis for the vector space

E_x

. Equivalently, a frame can be viewed as a linear isomorphism

p:R^k\toE_x.

The set of all frames at x

, denoted F_x

, has a natural right action by the general linear group GL(k,R)

of invertible k x k

matrices: a group element g\inGL(k,R)

acts on the frame p

via composition to give a new frame

p\circg:R^k\toE_x.

This action of GL(k,R)

on F_x

is both free and transitive (this follows from the standard linear algebra result that there is a unique invertible linear transformation sending one basis onto another). As a topological space, F_x

is homeomorphic to GL(k,R)

although it lacks a group structure, since there is no "preferred frame". The space F_x

is said to be a GL(k,R)

-torsor.

The frame bundle of

, denoted by
F(E)

or
F_GL(E)

, is the disjoint union of all the

F_x

F(E)=\coprod_x\inF_x.

Each point in

F(E)

is a pair (x, p) where x

is a point in X

and p

is a frame at x

. There is a natural projection

\pi:F(E)\toX

which sends (x,p)

to x

. The group GL(k,R)

acts on

F(E)

on the right as above. This action is clearly free and the orbits are just the fibers of \pi

.

Principal bundle structure

The frame bundle

F(E)

can be given a natural topology and bundle structure determined by that of E

. Let (U_i,\phi_i)

be a local trivialization of E

. Then for each x ∈ U_i one has a linear isomorphism \phi_i,x:E_x\toR^k

. This data determines a bijection

\psi_i:\pi^-1(U_i)\toU_{i x}GL(k,R)

given by

\psi_i(x,p)=(x,\varphi_i,x\circp).

With these bijections, each \pi^-1(U_i)

can be given the topology of U_i x GL(k,R)

. The topology on

F(E)

is the final topology coinduced by the inclusion maps \pi^-1(U_i)\toF(E)

.

With all of the above data the frame bundle

F(E)

becomes a principal fiber bundle over X

with structure group GL(k,R)

and local trivializations (\{U_i\},\{\psi_i\})

. One can check that the transition functions of

F(E)

are the same as those of E

.

The above all works in the smooth category as well: if

is a smooth vector bundle over a smooth manifold

then the frame bundle of

can be given the structure of a smooth principal bundle over

Associated vector bundles

A vector bundle

and its frame bundle
F(E)

are associated bundles. Each one determines the other. The frame bundle
F(E)

can be constructed from

as above, or more abstractly using the fiber bundle construction theorem. With the latter method,
F(E)

is the fiber bundle with same base, structure group, trivializing neighborhoods, and transition functions as

but with abstract fiber

GL(k,R)

, where the action of structure group

GL(k,R)

on the fiber

GL(k,R)

is that of left multiplication.

Given any linear representation

\rho:GL(k,R)\toGL(V,F)

there is a vector bundle

F(E) x _\rhoV

associated with

F(E)

which is given by product

F(E) x V

modulo the equivalence relation (pg,v)\sim(p,\rho(g)v)

for all g

in GL(k,R)

. Denote the equivalence classes by [p,v]

.

The vector bundle

is naturally isomorphic to the bundle
F(E) x _\rhoR^k

where \rho

is the fundamental representation of

GL(k,R)

on R^k

. The isomorphism is given by

[p,v]\mapstop(v)

where v

is a vector in R^k

and p:R^k\toE_x

is a frame at x

. One can easily check that this map is well-defined.

Any vector bundle associated with

can be given by the above construction. For example, the dual bundle of

is given by
F(E)

x
\rho^*

(R^k)^*

where
\rho^*

is the dual of the fundamental representation. Tensor bundles of

can be constructed in a similar manner.

Tangent frame bundle

The tangent frame bundle (or simply the frame bundle) of a smooth manifold

is the frame bundle associated with the tangent bundle of

. The frame bundle of

is often denoted

GL(M)

rather than

F(TM)

. In physics, it is sometimes denoted

. If

-dimensional then the tangent bundle has rank

, so the frame bundle of

is a principal

GL(n,R)

bundle over

Smooth frames

Local sections of the frame bundle of

are called smooth frames on

. The cross-section theorem for principal bundles states that the frame bundle is trivial over any open set in

which admits a smooth frame. Given a smooth frame

s:U\toFU

, the trivialization

\psi:FU\toU x GL(n,R)

is given by

\psi(p)=(x,s(x)^-1\circp)

where p

is a frame at x

. It follows that a manifold is parallelizable if and only if the frame bundle of M

admits a global section.

Since the tangent bundle of

is trivializable over coordinate neighborhoods of

so is the frame bundle. In fact, given any coordinate neighborhood

with coordinates

(x^1,\ldots,xⁿ⁾

the coordinate vector fields

\left(	\partial	,\ldots,
	\partialx¹

	\partial
	\partialxⁿ

\right)

define a smooth frame on U

. One of the advantages of working with frame bundles is that they allow one to work with frames other than coordinates frames; one can choose a frame adapted to the problem at hand. This is sometimes called the method of moving frames.

Solder form

The frame bundle of a manifold

is a special type of principal bundle in the sense that its geometry is fundamentally tied to the geometry of

. This relationship can be expressed by means of a vector-valued 1-form on

called the solder form (also known as the fundamental or tautological 1-form). Let

be a point of the manifold

and

a frame at

, so that

p:R^n\toT_xM

is a linear isomorphism of Rⁿ

with the tangent space of M

at x

. The solder form of FM

is the Rⁿ

-valued 1-form \theta

defined by

\theta_p(\xi)=p^-1d\pi(\xi)

where ξ is a tangent vector to FM

at the point (x,p)

, and p^-1:T_xM\toRⁿ

is the inverse of the frame map, and d\pi

is the differential of the projection map \pi:FM\toM

. The solder form is horizontal in the sense that it vanishes on vectors tangent to the fibers of \pi

and right equivariant in the sense that

	*\theta
R
	g

=g^-1\theta

where R_g

is right translation by g\inGL(n,R)

. A form with these properties is called a basic or tensorial form on FM

. Such forms are in 1-1 correspondence with TM

-valued 1-forms on M

which are, in turn, in 1-1 correspondence with smooth bundle maps TM\toTM

over M

. Viewed in this light \theta

is just the identity map on TM

.

As a naming convention, the term "tautological one-form" is usually reserved for the case where the form has a canonical definition, as it does here, while "solder form" is more appropriate for those cases where the form is not canonically defined. This convention is not being observed here.

Orthonormal frame bundle

If a vector bundle

is equipped with a Riemannian bundle metric then each fiber

E_x

is not only a vector space but an inner product space. It is then possible to talk about the set of all orthonormal frames for

E_x

. An orthonormal frame for

E_x

is an ordered orthonormal basis for

E_x

, or, equivalently, a linear isometry

p:R^k\toE_x

where R^k

is equipped with the standard Euclidean metric. The orthogonal group O(k)

acts freely and transitively on the set of all orthonormal frames via right composition. In other words, the set of all orthonormal frames is a right O(k)

-torsor.

The orthonormal frame bundle of

, denoted

F_O(E)

, is the set of all orthonormal frames at each point

in the base space

. It can be constructed by a method entirely analogous to that of the ordinary frame bundle. The orthonormal frame bundle of a rank

Riemannian vector bundle

E\toX

is a principal

O(k)

-bundle over

. Again, the construction works just as well in the smooth category.

If the vector bundle

is orientable then one can define the oriented orthonormal frame bundle of

, denoted

F_SO(E)

, as the principal

SO(k)

-bundle of all positively oriented orthonormal frames.

is an

-dimensional Riemannian manifold, then the orthonormal frame bundle of

, denoted

F_O(M)

O(M)

, is the orthonormal frame bundle associated with the tangent bundle of

(which is equipped with a Riemannian metric by definition). If

is orientable, then one also has the oriented orthonormal frame bundle

F_SOM

Given a Riemannian vector bundle

, the orthonormal frame bundle is a principal

O(k)

-subbundle of the general linear frame bundle. In other words, the inclusion map

i:{F}_O(E)\to{F}_GL(E)

is principal bundle map. One says that F_O(E)

is a reduction of the structure group of F_GL(E)

from GL(n,R)

to O(k)

.

G-structures

If a smooth manifold

comes with additional structure it is often natural to consider a subbundle of the full frame bundle of

which is adapted to the given structure. For example, if

is a Riemannian manifold we saw above that it is natural to consider the orthonormal frame bundle of

. The orthonormal frame bundle is just a reduction of the structure group of

F_GL(M)

to the orthogonal group

O(n)

In general, if

is a smooth

-manifold and

is a Lie subgroup of

GL(n,R)

we define a G-structure on

to be a reduction of the structure group of

F_GL(M)

. Explicitly, this is a principal

-bundle

F_G(M)

over

together with a

-equivariant bundle map

{F}_G(M)\to{F}_GL(M)

over M

.

In this language, a Riemannian metric on

gives rise to an

O(n)

-structure on

. The following are some other examples.

Every oriented manifold has an oriented frame bundle which is just a

GL^+(n,R)

-structure on

A volume form on

determines a

SL(n,R)

-structure on

-dimensional symplectic manifold has a natural

Sp(2n,R)

-structure.

-dimensional complex or almost complex manifold has a natural

GL(n,C)

-structure.In many of these instances, a

-structure on

uniquely determines the corresponding structure on

. For example, a

SL(n,R)

-structure on

determines a volume form on

. However, in some cases, such as for symplectic and complex manifolds, an added integrability condition is needed. A

Sp(2n,R)

-structure on

uniquely determines a nondegenerate 2-form on

, but for

to be symplectic, this 2-form must also be closed.
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