# Exterior covariant derivative

In mathematics, the exterior covariant derivative, sometimes also covariant exterior derivative, is a very useful notion for calculus on manifolds, which makes it possible to simplify formulas which use a principal connection.

## Definition

Let PM be a principal G-bundle on a smooth manifold M. If ϕ is a tensorial k-form on P, then its exterior covariant derivative is defined by

$D\phi(X_0,X_1,\dots,X_k)=\mathrm{d}\phi(h(X_0),h(X_1),\dots,h(X_k))$

where h denotes the projection to the horizontal subspace, Hx defined by the connection, with kernel Vx (the vertical subspace) of the tangent bundle of the total space of the fiber bundle. Here Xi are any vector fields on P. Dϕ is a tensorial (k + 1)-form on P.

## Properties

Unlike the usual exterior derivative, which squares to 0 (that is d2 = 0), we have

$D^2\phi=\Omega\wedge\phi$

where Ω denotes the curvature form. In particular D2 vanishes for a flat connection.