# Connection (mathematics)

In differential geometry, a connection (also connexion) or covariant derivative is a way of specifying a derivative of a vector field along another vector field on a manifold. That is an application to tangent bundles; there are more general connections, used in differential geometry and other fields of mathematics to formulate intrinsic differential equations . Connection may refer to a connection on any vector bundle, or also a connection on a principal bundle.

Connections give rise to parallel transport along a curve on a manifold. A connection also leads to invariants of curvature (see also curvature tensor and curvature form), and the so-called torsion.

## General concept

The general concept can be summarized as follows: given a fiber bundle [itex]\eta:E\to B[itex] the tangent space at any point of E has canonical "vertical" subspace, the subspace tangent to the fiber. The connection fixes a choice of "horizontal" subspace at each point of E so that the tangent space of E is a direct sum of vertical and horizontal subspaces. Usually more requirements are imposed on the choice of "horizontal" subspaces, but they depend on the type of the bundle.

Given a [itex]B'\to B[itex] the induced bundle has an induced connection. If [itex]B'=I[itex] is a segment then connection on B gives a trivialization on the induced bundle over I, i.e. a choice of smooth one-parameter family of isomorphisms between the fibers over I. This family is called parallel displacement along the curve [itex]I\to B [itex] and it gives an equivalent description of connection (which in case of Levi-Civita connection on a Riemannian manifold is called parallel transport).

There are many ways to describe connection, in one particular approach, a connection can be locally described as a matrix of 1-forms which is the multiplant of the difference between the covariant derivative and the ordinary partial derivative in a coordinate chart. That is, partial derivatives are not an intrinsic notion on a manifold: a connection 'fixes up' the concept and permits discussion in geometric terms.

## Possible approaches

There are quite a number of possible approaches to the connection concept. They include the following:

The connections referred to above are linear or affine connections. There is also a concept of projective connection; the most commonly-met form of this is the Schwarzian derivative in complex analysis.

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