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## Summary

In differential geometry, the curvature form describes curvature of a connection on a principal bundle. The Riemann curvature tensor in Riemannian geometry can be considered as a special case.

## Definition

Let G be a Lie group with Lie algebra ${\mathfrak {g}}$ , and PB be a principal G-bundle. Let ω be an Ehresmann connection on P (which is a ${\mathfrak {g}}$ -valued one-form on P).

Then the curvature form is the ${\mathfrak {g}}$ -valued 2-form on P defined by

$\Omega =d\omega +{1 \over 2}[\omega \wedge \omega ]=D\omega .$

(In another convention, 1/2 does not appear.) Here $d$  stands for exterior derivative, $[\cdot \wedge \cdot ]$  is defined in the article "Lie algebra-valued form" and D denotes the exterior covariant derivative. In other terms,

$\,\Omega (X,Y)=d\omega (X,Y)+{1 \over 2}[\omega (X),\omega (Y)]$

where X, Y are tangent vectors to P.

There is also another expression for Ω: if X, Y are horizontal vector fields on P, then

$\sigma \Omega (X,Y)=-\omega ([X,Y])=-[X,Y]+h[X,Y]$

where hZ means the horizontal component of Z, on the right we identified a vertical vector field and a Lie algebra element generating it (fundamental vector field), and $\sigma \in \{1,2\}$  is the inverse of the normalization factor used by convention in the formula for the exterior derivative.

A connection is said to be flat if its curvature vanishes: Ω = 0. Equivalently, a connection is flat if the structure group can be reduced to the same underlying group but with the discrete topology.

### Curvature form in a vector bundle

If EB is a vector bundle, then one can also think of ω as a matrix of 1-forms and the above formula becomes the structure equation of E. Cartan:

$\,\Omega =d\omega +\omega \wedge \omega ,$

where $\wedge$  is the wedge product. More precisely, if ${\omega ^{i}}_{j}$  and ${\Omega ^{i}}_{j}$  denote components of ω and Ω correspondingly, (so each ${\omega ^{i}}_{j}$  is a usual 1-form and each ${\Omega ^{i}}_{j}$  is a usual 2-form) then

$\Omega _{j}^{i}=d{\omega ^{i}}_{j}+\sum _{k}{\omega ^{i}}_{k}\wedge {\omega ^{k}}_{j}.$

For example, for the tangent bundle of a Riemannian manifold, the structure group is O(n) and Ω is a 2-form with values in the Lie algebra of O(n), i.e. the antisymmetric matrices. In this case the form Ω is an alternative description of the curvature tensor, i.e.

$\,R(X,Y)=\Omega (X,Y),$

using the standard notation for the Riemannian curvature tensor.

## Bianchi identities

If $\theta$  is the canonical vector-valued 1-form on the frame bundle, the torsion $\Theta$  of the connection form $\omega$  is the vector-valued 2-form defined by the structure equation

$\Theta =d\theta +\omega \wedge \theta =D\theta ,$

where as above D denotes the exterior covariant derivative.

The first Bianchi identity takes the form

$D\Theta =\Omega \wedge \theta .$

The second Bianchi identity takes the form

$\,D\Omega =0$

and is valid more generally for any connection in a principal bundle.

The contracted Bianchi identities are used to derive the Einstein tensor in the Einstein field equations, the bulk of general theory of relativity.