# Curvature form

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 P → B 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 E → B 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 Bianchi identities can be written in tensor notation as: $R_{abmn;\ell }+R_{ab\ell m;n}+R_{abn\ell ;m}=0.$
The contracted Bianchi identities are used to derive the Einstein tensor in the Einstein field equations, a key component in the general theory of relativity.