# Connection form

In mathematics, and specifically differential geometry, the connection form captures the invariant aspects of the connection on principal bundles, vector bundles and line bundles. In a certain sense, it captures the idea of Christoffel symbols on a Riemannian manifold and re-expresses this idea in a more general way, so that its applicable on a principle bundle. See also Cartan connection for a more abstract treatment.

 Contents

## Principal bundles

For a principal G-bundle [itex]E\to B [itex], for each [itex]x\in E[itex] let [itex]T_x(E)[itex] denote the tangent space at x and [itex]V_x[itex] the vertical subspace tangent to the fiber . Then connection is an assignment of a horizontal subspace [itex]H_x[itex] of [itex]T_x(E)[itex] such that

1. [itex]T_x(E)[itex] is direct sum of [itex]V_x[itex] and [itex]H_x[itex],
2. The distribution of [itex]H_x[itex] is invariant with respect to the G-action on E, i.e. [itex]H_{ax}=D_x(R_a)H_{x}[itex] for any [itex]x\in E[itex] and [itex]a\in G[itex], here [itex]D_x(R_a)[itex] denotes the differential of the group action by a at x.
3. The distribution [itex]H_x[itex] depends smoothly on x.

This can be recast more elegantly using the jet bundle [itex]JE \rightarrow E[itex]. The assignment of a horizontal subspace at each point is none other than a smooth section of this jet bundle.

The subspace [itex]V_x[itex] can be naturally identified with the Lie algebra g of group G, say by map [itex]\iota:V_x\to g[itex]. Then the connection form is a form [itex]\omega[itex] on [itex]E[itex] with values in g defined by [itex]\omega(X)=\iota\circ v(X)[itex] where [itex]v[itex] denotes projection at [itex]x\in E[itex] of [itex]X \in T_x[itex] to [itex]V_x[itex] with kernel [itex]H_x[itex].

Given a local trivialization one can reduce [itex]\omega[itex] to the horizontal vector fields (in this trivialization). It defines form say [itex]\omega'[itex] on B. The form [itex]\omega'[itex] defines [itex]\omega[itex] completely, but it depends on the choice of trivialization. (This form is often also called connection form and denoted also by [itex]\omega.[itex])

### Related definitions

#### Exterior covariant derivative

The exterior covariant derivative is a very useful notion which makes possible to simplify formulas in using connection. Given a tensor-valued differential k-form [itex]\phi [itex] its exterior covariant derivative defined by

[itex]D\phi(X_0,X_1,...,X_k)=d\phi(h(X_0),h(X_1),...,h(X_k))[itex]

where h denotes the projection to the horizontal subspace, [itex]H_x[itex] with kernel [itex]V_x[itex] and [itex]X_i[itex] are arbitrary vector fields on E.

#### Curvature form

The curvature form [itex]\Omega[itex], a g-valued 2-form, can be defined by

[itex]\Omega=d\omega +{1\over 2}[\omega,\omega]=D\omega,[itex]

where [itex][*,*][itex] denotes the Lie bracket. This equation is also called the second structure equation.

#### Torsion

For the connection on a frame bundle, the curvature is not the only invariant of connection since the additional structure should be taken into account. Namely one has an extra canonical Rn-valued form [itex]\theta=\theta^i[itex] on E defined by identity [itex] X=\sum_i\theta^i(X)e_i.[itex]. Then the torsion form, an Rn-valued 2-form can be defined by

[itex] \Theta=d\theta+{1\over 2}[\omega, \theta]=D\theta. [itex]

This equation is also called the first structure equation.

## Vector bundles

The connection form for the vector bundle is the form on the total space of associated principal bundle, but it can be completely described by the following form (on the base in a NOT invariant way). This subsection can be considered as a smoother but bit wrong introduction to connection form.

A covariant derivative on a vector bundle is a way to "differentiate" bundle sections along tangent vectors, it is also sometimes called connection. Let [itex]\zeta:E\to B[itex] be a vector bundle over a smooth manifold [itex]B[itex] with a n-dimensional vector space [itex]F[itex] as a fiber. Let us denote by [itex]\nabla_uv[itex] a section of the vector bundle, the result of differentiation of the section of vector bundle [itex]v[itex] along tangent vector field [itex]u[itex]. In order to be a covariant derivative [itex]\nabla[itex] must satisfy the following identities:

(i) [itex]\nabla_u(v_1+v_2)=\nabla_uv_1+\nabla_uv_2[itex] and [itex]\nabla_{u_1+u_2}v=\nabla_{u_1}v+\nabla_{u_2}v[itex] (linearity)
(ii) [itex]\nabla_u(fv)=df(u) v +f\nabla_uv [itex] and [itex]\nabla_{f u}v=f\nabla_{u}v[itex] for any smooth function [itex]f.[itex]

The simplest example: if [itex]\zeta:E=F\times B \to B[itex] is the projection, i.e. [itex]\zeta[itex] is a trivial vector bundle, then any section can be described by a smooth map [itex]v:B\to F[itex]. Therefore one can consider the trivial covariant derivative defined by partial derivatives: [itex]\nabla_u v=\partial v/\partial u.[itex]

If one has two connections [itex]\nabla[itex] and [itex]\nabla'[itex] on the same vector bundle then the difference [itex]\omega(u)v=\nabla_uv-\nabla'_uv[itex] depends only on values of u and v at a point, [itex]\omega[itex] is a 1-form on [itex]B[itex] with values in [itex]Hom(F,F)[itex]; i.e. [itex]\omega(u)\in Hom(F,F)[itex] and [itex]\omega[itex] can be described as an [itex]n\times n[itex]-matrix of one-forms. In particular one can choose a local trivialization of the vector bundle and take [itex]\nabla'[itex] to be correspondent trivial connection, then [itex]\omega[itex] gives a complete local description of [itex]\nabla[itex].

The choice of trivialization is equivalent to choice of frames in each fiber, that explains the reason for the name Method of moving frames. Let us choose (a local smooth section of) basis frames [itex]e_i[itex] in fibers. Then the matrix of 1-forms [itex]\omega=\omega_i^j[itex] is defined by the following identity:

[itex]\nabla_u e_i=\sum_j\omega^j_i(u)e_j.[itex]

If [itex]G\in GL(F)[itex] is the structure group of the vector bundle and connection [itex]\nabla[itex] respects... the group then the form [itex]\omega[itex] is a 1-form with values in [itex]g[itex], the Lie algebra of [itex]G[itex]. In particular for the tangent bundle of a Riemannian manifold we have [itex]O(n)[itex] as the structure group and for the form [itex]\omega[itex] for the Levi-Civita connection is a form with values in so(n), the Lie algebra of [itex]O(n)[itex] (which can be thought of as antisymmetric matrices in an orthonormal basis).

### Related definitions

#### Curvature

The connection form ([itex]\omega[itex]) describes connection ([itex]\nabla[itex]) in a non-invariant way; it depends on the choice of local trivialization. The following construction extracts invariant information out of [itex]\omega[itex].

The following 2-form with values in [itex]Hom(F,F)[itex] is called curvature form

[itex]\Omega=d\omega +\omega\wedge\omega,[itex]

where [itex]d[itex] stands for exterior derivative and [itex]\wedge[itex] is the wedge product. This equation also called the second structure equation.

#### Torsion

For the connection on tangent bundle the curvature is not the only invariant of connection since the additional structure should be taken into account. Namely one has an extra canonical Rn-valued form [itex]\theta=\theta^i[itex] on B defined by identity [itex] X=\sum_i\theta^i(X)e_i.[itex]

Then the torsion, an of Rn-valued 2-form can be defined by

[itex] \Theta=d\theta+\omega\wedge \theta\ \ \mbox{or} \ \ \Theta^i=d\theta^i+\sum_j\omega^i_j\wedge \theta^j.[itex]

This equation is also called the first structure equation.

## References

• Kobayashi, Shoshichi; Nomizu, Katsumi Foundations of differential geometry. Vol. I. Reprint of the 1963 original. Wiley Classics Library. A Wiley-Interscience Publication. John Wiley & Sons, Inc., New York, 1996. xii+329 pp. ISBN 0-471-15733-3

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