In mathematics, and especially differential geometry and gauge theory, a **connection** on a fiber bundle is a device that defines a notion of parallel transport on the bundle; that is, a way to "connect" or identify fibers over nearby points. The most common case is that of a **linear connection** on a vector bundle, for which the notion of parallel transport must be linear. A linear connection is equivalently specified by a *covariant derivative*, an operator that differentiates sections of the bundle along tangent directions in the base manifold, in such a way that parallel sections have derivative zero. Linear connections generalize, to arbitrary vector bundles, the Levi-Civita connection on the tangent bundle of a pseudo-Riemannian manifold, which gives a standard way to differentiate vector fields. Nonlinear connections generalize this concept to bundles whose fibers are not necessarily linear.

Linear connections are also called **Koszul connections** after Jean-Louis Koszul, who gave an algebraic framework for describing them (Koszul 1950).

This article defines the connection on a vector bundle using a common mathematical notation which de-emphasizes coordinates. However, other notations are also regularly used: in general relativity, vector bundle computations are usually written using indexed tensors; in gauge theory, the endomorphisms of the vector space fibers are emphasized. The different notations are equivalent, as discussed in the article on metric connections (the comments made there apply to all vector bundles).

## Motivation

A section of a vector bundle generalises the notion of a function on a manifold, in the sense that a standard vector-valued function can be viewed as a section of the trivial vector bundle . It is therefore natural to ask if it is possible to differentiate a section in analogy to how one differentiates a vector field. When the vector bundle is the tangent bundle to a pseudo-Riemannian manifold, this question is answered naturally by the Levi-Civita connection, which is the unique torsion-free connection compatible with the pseudo-Riemannian metric on the tangent bundle. In general there is no such natural choice of a way to differentiate sections.

The model case is to differentiate an -component vector field on Euclidean space . In this setting the derivative at a point in the direction may be simply defined by

Notice that for every , we have defined a new vector so the derivative of in the direction of has yielded a new -component vector field on .

When passing to a section of a vector bundle on a manifold , one encounters two key issues with this definition. Firstly, since the manifold has no linear structure, the term makes no sense on . Instead one takes a path such that and computes

However this still does not make sense, because is a vector in the fibre over , and , the fibre over , which is a different vector space. This means there is no way to make sense of the subtraction of these two terms lying in different vector spaces.

The goal is to resolve the above conundrum by coming up with a way of differentiating sections of a vector bundle in the direction of vector fields, and getting back another section of the vector bundle. There are three possible resolutions to this problem. All three require making a *choice* of how to differentiate sections, and only in special settings like the tangent bundle on a Riemannian manifold is there a natural such choice.

- (
*Parallel transport*) Since the problem is that the vectors and lie in different fibres of , one solution is to define an isomorphism for all close to zero. Using this isomorphism one can transport to the fibre and then take the difference. Explicitly,This is the parallel transport, and the choice of the isomorphisms for all curves in can be taken as the definition of how to differentiate a section. - (
*Ehresmann connection*) Use the notion of differential of a map of smooth manifolds. A section is by definition a smooth map such that . This has a differential , with the property that for a vector field . However, one would like instead for to be a section of itself. In fact, the vertical bundle is the pullback of along with the same fiber as . If one chooses a projection of vector bundles, composing with this projection would land back in . This is called a linear Ehresmann connection on the vector bundle . There are many choices of projection operators so in general there are many different ways of differentiating a vector field. - (
*Covariant derivative*) The third solution is to abstract the properties that a derivative of a section of a vector bundle should have and take this as an axiomatic definition. This is the notion of a**connection**or**covariant derivative**described in this article. The other two approaches above can both be shown to be equivalent to this axiomatic definition of differentiation.

## Formal definition

Let be a smooth vector bundle over a differentiable manifold . Denote the space of smooth sections of by . A **connection** on is an -linear map (or when the vector bundle is a complex vector bundle, a -linear map)

such that the Leibniz rule

holds for all smooth functions on and all smooth sections of .

If is a tangent vector field on (i.e. a section of the tangent bundle ) one can define a **covariant derivative along **

by contracting with the resulting covariant index in the connection: . The covariant derivative satisfies:

Conversely, any operator satisfying the above properties defines a connection on and a connection in this sense is also known as a **covariant derivative** on .

## Induced connections

Given a vector bundle , there are many associated bundles to which may be constructed, for example the dual vector bundle , tensor powers , symmetric and antisymmetric tensor powers , and the direct sums . A connection on induces a connection on any one of these associated bundles. The ease of passing between connections on associated bundles is more elegantly captured by the theory of principal bundle connections, but here we present some of the basic induced connections.

### Dual connection

Given a connection on , the induced **dual connection** on is defined implicitly by

Here is a smooth vector field, is a section of , and a section of the dual bundle, and the natural pairing between a vector space and its dual (occurring on each fibre between and ). Notice that this definition is essentially enforcing that be the connection on so that a natural product rule is satisfied for pairing .

### Tensor product connection

Given connections on two vector bundles , define the **tensor product connection** by the formula

Here we have . Notice again this is the natural way of combining to enforce the product rule for the tensor product connection. By repeated application of the above construction applied to the tensor product , one also obtains the **tensor power connection** on for any and vector bundle .

### Direct sum connection

The **direct sum connection** is defined by

where .

### Symmetric and exterior power connections

Since the symmetric power and exterior power of a vector bundle may be viewed naturally as subspaces of the tensor power, , the definition of the tensor product connection applies in a straightforward manner to this setting. Indeed, since the symmetric and exterior algebras sit inside the tensor algebra as direct summands, and the connection respects this natural splitting, one can simply restrict to these summands. Explicitly, define the **symmetric product connection** by

and the **exterior product connection** by

for all . Repeated applications of these products gives induced **symmetric power** and **exterior power connections** on and respectively.

### Endomorphism connection

Finally, one may define the induced connection on the vector bundle of endomorphisms , the **endomorphism connection**. This is simply the tensor product connection of the dual connection on and on . If and , so that the composition also, then the following product rule holds for the endomorphism connection:

By reversing this equation, it is possible to define the endomorphism connection as the unique connection satisfying

for any , thus avoiding the need to first define the dual connection and tensor product connection.

### Any associated bundle

Given a vector bundle of rank , and any representation into a linear group , there is an induced connection on the associated vector bundle . This theory is most succinctly captured by passing to the principal bundle connection on the frame bundle of and using the theory of principal bundles. Each of the above examples can be seen as special cases of this construction: the dual bundle corresponds to the inverse transpose (or inverse adjoint) representation, the tensor product to the tensor product representation, the direct sum to the direct sum representation, and so on.

## Exterior covariant derivative and vector-valued forms

Let be a vector bundle. An -valued differential form of degree is a section of the tensor product bundle:

The space of such forms is denoted by

where the last tensor product denotes the tensor product of modules over the ring of smooth functions on .

An -valued 0-form is just a section of the bundle . That is,

In this notation a connection on is a linear map

A connection may then be viewed as a generalization of the exterior derivative to vector bundle valued forms. In fact, given a connection on there is a unique way to extend to an **exterior covariant derivative**

This exterior covariant derivative is defined by the following Leibniz rule, which is specified on simple tensors of the form and extended linearly:

where so that , is a section, and denotes the -form with values in defined by wedging with the one-form part of . Notice that for -valued 0-forms, this recovers the normal Leibniz rule for the connection .

Unlike the ordinary exterior derivative, one generally has . In fact, is directly related to the curvature of the connection (see below).

## Affine properties of the set of connections

Every vector bundle over a manifold admits a connection, which can be proved using partitions of unity. However, connections are not unique. If and are two connections on then their difference is a -linear operator. That is,

for all smooth functions on and all smooth sections of . It follows that the difference can be uniquely identified with a one-form on with values in the endomorphism bundle :

Conversely, if is a connection on and is a one-form on with values in , then is a connection on .

In other words, the space of connections on is an affine space for . This affine space is commonly denoted .

## Relation to principal and Ehresmann connections

Let be a vector bundle of rank and let be the principal frame bundle of . Then a (principal) connection on induces a connection on . First note that sections of are in one-to-one correspondence with right-equivariant maps . (This can be seen by considering the pullback of over , which is isomorphic to the trivial bundle .) Given a section of let the corresponding equivariant map be . The covariant derivative on is then given by

where is the horizontal lift of from to . (Recall that the horizontal lift is determined by the connection on .)

Conversely, a connection on determines a connection on , and these two constructions are mutually inverse.

A connection on is also determined equivalently by a linear Ehresmann connection on . This provides one method to construct the associated principal connection.

The induced connections discussed in #Induced connections can be constructed as connections on other associated bundles to the frame bundle of , using representations other than the standard representation used above. For example if denotes the standard representation of on , then the associated bundle to the representation of on is the direct sum bundle , and the induced connection is precisely that which was described above.

## Local expression

Let be a vector bundle of rank , and let be an open subset of over which trivialises. Therefore over the set , admits a local smooth frame of sections

Since the frame defines a basis of the fibre for any , one can expand any local section in the frame as

for a collection of smooth functions .

Given a connection on , it is possible to express over in terms of the local frame of sections, by using the characteristic product rule for the connection. For any basis section , the quantity may be expanded in the local frame as

where are a collection of local one-forms. These forms can be put into a matrix of one-forms defined by

called the *local connection form of over *. The action of on any section can be computed in terms of using the product rule as

If the local section is also written in matrix notation as a column vector using the local frame as a basis,

then using regular matrix multiplication one can write

where is shorthand for applying the exterior derivative to each component of as a column vector. In this notation, one often writes locally that . In this sense a connection is locally completely specified by its connection one-form in some trivialisation.

As explained in #Affine properties of the set of connections, any connection differs from another by an endomorphism-valued one-form. From this perspective, the connection one-form is precisely the endomorphism-valued one-form such that the connection on differs from the trivial connection on , which exists because is a trivialising set for .

### Relationship to Christoffel symbols

In pseudo-Riemannian geometry, the Levi-Civita connection is often written in terms of the Christoffel symbols instead of the connection one-form . It is possible to define Christoffel symbols for a connection on any vector bundle, and not just the tangent bundle of a pseudo-Riemannian manifold. To do this, suppose that in addition to being a trivialising open subset for the vector bundle , that is also a local chart for the manifold , admitting local coordinates .

In such a local chart, there is a distinguished local frame for the differential one-forms given by , and the local connection one-forms can be expanded in this basis as

for a collection of local smooth functions , called the *Christoffel symbols* of over . In the case where and is the Levi-Civita connection, these symbols agree precisely with the Christoffel symbols from pseudo-Riemannian geometry.

The expression for how acts in local coordinates can be further expanded in terms of the local chart and the Christoffel symbols, to be given by

Contracting this expression with the local coordinate tangent vector leads to

This defines a collection of locally defined operators

with the property that

### Change of local trivialisation

Suppose is another choice of local frame over the same trivialising set , so that there is a matrix of smooth functions relating and , defined by

Tracing through the construction of the local connection form for the frame , one finds that the connection one-form for is given by

where denotes the inverse matrix to . In matrix notation this may be written

where is the matrix of one-forms given by taking the exterior derivative of the matrix component-by-component.

In the case where is the tangent bundle and is the Jacobian of a coordinate transformation of , the lengthy formulae for the transformation of the Christoffel symbols of the Levi-Civita connection can be recovered from the more succinct transformation laws of the connection form above.

## Parallel transport and holonomy

A connection on a vector bundle defines a notion of parallel transport on along a curve in . Let be a smooth path in . A section of along is said to be **parallel** if

for all . Equivalently, one can consider the pullback bundle of by . This is a vector bundle over with fiber over . The connection on pulls back to a connection on . A section of is parallel if and only if .

Suppose is a path from to in . The above equation defining parallel sections is a first-order ordinary differential equation (cf. local expression above) and so has a unique solution for each possible initial condition. That is, for each vector in there exists a unique parallel section of with . Define a **parallel transport map**

by . It can be shown that is a linear isomorphism, with inverse given by following the same procedure with the reversed path from to .

Parallel transport can be used to define the holonomy group of the connection based at a point in . This is the subgroup of consisting of all parallel transport maps coming from loops based at :

The holonomy group of a connection is intimately related to the curvature of the connection (AmbroseSinger 1953).

The connection can be recovered from its parallel transport operators as follows. If is a vector field and a section, at a point pick an integral curve for at . For each we will write for the parallel transport map traveling along from to . In particular for every , we have . Then defines a curve in the vector space , which may be differentiated. The covariant derivative is recovered as

This demonstrates that an equivalent definition of a connection is given by specifying all the parallel transport isomorphisms between fibres of and taking the above expression as the definition of .

## Curvature

The **curvature** of a connection on is a 2-form on with values in the endomorphism bundle . That is,

It is defined by the expression

where and are tangent vector fields on and is a section of . One must check that is -linear in both and and that it does in fact define a bundle endomorphism of .

As mentioned above, the covariant exterior derivative need not square to zero when acting on -valued forms. The operator is, however, strictly tensorial (i.e. -linear). This implies that it is induced from a 2-form with values in . This 2-form is precisely the curvature form given above. For an -valued form we have

A **flat connection** is one whose curvature form vanishes identically.

### Local form and Cartan's structure equation

The curvature form has a local description called **Cartan's structure equation**. If has local form on some trivialising open subset for , then

on . To clarify this notation, notice that is a endomorphism-valued one-form, and so in local coordinates takes the form of a matrix of one-forms. The operation applies the exterior derivative component-wise to this matrix, and denotes matrix multiplication, where the components are wedged rather than multiplied.

In local coordinates on over , if the connection form is written for a collection of local endomorphisms , then one has

Further expanding this in terms of the Christoffel symbols produces the familiar expression from Riemannian geometry. Namely if is a section of over , then

Here is the full **curvature tensor** of , and in Riemannian geometry would be identified with the Riemannian curvature tensor.

It can be checked that if we define to be wedge product of forms but commutator of endomorphisms as opposed to composition, then , and with this alternate notation the Cartan structure equation takes the form

This alternate notation is commonly used in the theory of principal bundle connections, where instead we use a connection form , a Lie algebra-valued one-form, for which there is no notion of composition (unlike in the case of endomorphisms), but there is a notion of a Lie bracket.

In some references (see for example (MadsenTornehave1997)) the Cartan structure equation may be written with a minus sign:

This different convention uses an order of matrix multiplication that is different from the standard Einstein notation in the wedge product of matrix-valued one-forms.

### Bianchi identity

A version of the second (differential) Bianchi identity from Riemannian geometry holds for a connection on any vector bundle. Recall that a connection on a vector bundle induces an endomorphism connection on . This endomorphism connection has itself an exterior covariant derivative, which we ambiguously call . Since the curvature is a globally defined -valued two-form, we may apply the exterior covariant derivative to it. The **Bianchi identity** says that

- .

This succinctly captures the complicated tensor formulae of the Bianchi identity in the case of Riemannian manifolds, and one may translate from this equation to the standard Bianchi identities by expanding the connection and curvature in local coordinates.

There is no analogue in general of the *first* (algebraic) Bianchi identity for a general connection, as this exploits the special symmetries of the Levi-Civita connection. Namely, one exploits that the vector bundle indices of in the curvature tensor may be swapped with the cotangent bundle indices coming from after using the metric to lower or raise indices. For example this allows the torsion-freeness condition to be defined for the Levi-Civita connection, but for a general vector bundle the -index refers to the local coordinate basis of , and the -indices to the local coordinate frame of and coming from the splitting . However in special circumstance, for example when the rank of equals the dimension of and a solder form has been chosen, one can use the soldering to interchange the indices and define a notion of torsion for affine connections which are not the Levi-Civita connection.

## Gauge transformations

Given two connections on a vector bundle , it is natural to ask when they might be considered equivalent. There is a well-defined notion of an automorphism of a vector bundle . A section is an automorphism if is invertible at every point . Such an automorphism is called a **gauge transformation** of , and the group of all automorphisms is called the **gauge group**, often denoted or . The group of gauge transformations may be neatly characterised as the space of sections of the *capital A adjoint bundle* of the frame bundle of the vector bundle . This is not to be confused with the *lowercase a adjoint bundle* , which is naturally identified with itself. The bundle is the associated bundle to the principal frame bundle by the conjugation representation of on itself, , and has fibre the same general linear group where . Notice that despite having the same fibre as the frame bundle and being associated to it, is not equal to the frame bundle, nor even a principal bundle itself. The gauge group may be equivalently characterised as

A gauge transformation of acts on sections , and therefore acts on connections by conjugation. Explicitly, if is a connection on , then one defines by

for . To check that is a connection, one verifies the product rule

It may be checked that this defines a left group action of on the affine space of all connections .

Since is an affine space modelled on , there should exist some endomorphism-valued one-form such that . Using the definition of the endomorphism connection induced by , it can be seen that

which is to say that .

Two connections are said to be **gauge equivalent** if they differ by the action of the gauge group, and the quotient space is the moduli space of all connections on . In general this topological space is neither a smooth manifold or even a Hausdorff space, but contains inside it the moduli space of Yang–Mills connections on , which is of significant interest in gauge theory and physics.

## Examples

- A classical covariant derivative or affine connection defines a connection on the tangent bundle of
*M*, or more generally on any tensor bundle formed by taking tensor products of the tangent bundle with itself and its dual. - A connection on can be described explicitly as the operator

- where is the exterior derivative evaluated on vector-valued smooth functions and are smooth. A section may be identified with a map
- and then

- If the bundle is endowed with a bundle metric, an inner product on its vector space fibers, a metric connection is defined as a connection that is compatible with the bundle metric.
- A Yang-Mills connection is a special metric connection which satisfies the Yang-Mills equations of motion.
- A Riemannian connection is a metric connection on the tangent bundle of a Riemannian manifold.
- A Levi-Civita connection is a special Riemannian connection: the metric-compatible connection on the tangent bundle that is also torsion-free. It is unique, in the sense that given any Riemannian connection, one can always find one and only one equivalent connection that is torsion-free. "Equivalent" means it is compatible with the same metric, although the curvature tensors may be different; see teleparallelism. The difference between a Riemannian connection and the corresponding Levi-Civita connection is given by the contorsion tensor.
- The exterior derivative is a flat connection on (the trivial line bundle over
*M*). - More generally, there is a canonical flat connection on any flat vector bundle (i.e. a vector bundle whose transition functions are all constant) which is given by the exterior derivative in any trivialization.

## See also

## References

- Chern, Shiing-Shen (1951),
*Topics in Differential Geometry*, Institute for Advanced Study, mimeographed lecture notes - Darling, R. W. R. (1994),
*Differential Forms and Connections*, Cambridge, UK: Cambridge University Press, ISBN 0-521-46800-0 - Kobayashi, Shoshichi; Nomizu, Katsumi (1996) [1963],
*Foundations of Differential Geometry, Vol. 1*, Wiley Classics Library, New York: Wiley Interscience, ISBN 0-471-15733-3 - Koszul, J. L. (1950), "Homologie et cohomologie des algebres de Lie",
*Bulletin de la Société Mathématique*,**78**: 65–127 - Wells, R.O. (1973),
*Differential analysis on complex manifolds*, Springer-Verlag, ISBN 0-387-90419-0 - Ambrose, W.; Singer, I.M. (1953), "A theorem on holonomy",
*Transactions of the American Mathematical Society*,**75**: 428–443, doi:10.2307/1990721

- Donaldson, S.K. and Kronheimer, P.B., 1997. The geometry of four-manifolds. Oxford University Press.

- Tu, L.W., 2017. Differential geometry: connections, curvature, and characteristic classes (Vol. 275). Springer.

- Taubes, C.H., 2011. Differential geometry: Bundles, connections, metrics and curvature (Vol. 23). OUP Oxford.

- Lee, J.M., 2018. Introduction to Riemannian manifolds. Springer International Publishing.
- Madsen, I.H.; Tornehave, J. (1997),
*From calculus to cohomology: de Rham cohomology and characteristic classes*, Cambridge University Press