# Exterior derivative

In mathematics, the exterior derivative operator of differential topology, extends the concept of the differential of a function to differential forms of higher degree. It is important in the theory of integration on manifolds, and is the differential used to define de Rham and Alexander-Spanier cohomology. Its current form was invented by Élie Cartan.

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## Definition

The exterior derivative of a differential form of degree k is a differential form of degree k + 1.

For a k-form ω = fI dxI over Rn, the definition is as follows:

[itex]d{\omega} = \sum_{i=1}^n \frac{\partial f_I}{\partial x_i} dx_i \wedge dx_I.[itex]

For general k-forms ΣI fI dxI (where the multi-index I runs over all ordered subsets of {1, ..., n} of cardinality k), we just extend linearly. Note that if [itex]i = I[itex] above then [itex]dx_i \wedge dx_I = 0[itex] (see wedge product).

## Properties

Exterior differentiation satisfies three important properties:

[itex]d(\omega \wedge \eta) = d\omega \wedge \eta+(-1)^{{\rm deg\,}\omega}(\omega \wedge d\eta)[itex]
[itex]d(d\omega)=0 \, \![itex]

It can be shown that exterior derivative is uniquely determined by these properties and its agreement with the differential on 0-forms (functions).

The kernel of d consists of the closed forms, and the image of the exact forms (cf. exact differentials).

## Invariant formula

Given a k-form ω and arbitrary smooth vector fields V0,V1, …, Vk we have

[itex]d\omega(V_0,V_1,...V_k)=\sum_i(-1)^i V_i\omega(V_0,...,\hat V_i,...,V_k)[itex]
[itex]+\sum_{i

where [itex][V_i,V_j][itex] denotes Lie bracket and the hat denotes the ommission of that element: [itex]\omega(V_0,...,\hat V_i,...,V_k)=\omega(V_0,..., V_{i-1},V_{i+1}...,V_k).[itex]

In particular, for 1-forms we have:

[itex]d\omega(X,Y)=X(\omega(Y))-Y(\omega(X))-\omega([X,Y]).[itex]

More generally, the Lie derivative is defined via the Lie bracket:

[itex]\mathcal{L}_XY=[X,Y][itex],

and the Lie derivative of a general differential form is closely related to the exterior derivative. The differences are primarily notational; various identities between the two are provided in the article on Lie derivatives.

## Connection with vector calculus

The following correspondence reveals about a dozen formulas from vector calculus as merely special cases of the above three rules of exterior differentiation.

For a 0-form, that is a smooth function f: RnR, we have

[itex]df = \sum_{i=1}^n \frac{\partial f}{\partial x_i}\, dx_i.[itex]

Therefore

where grad f denotes gradient of f and <•, •> is the scalar product.

### Curl

For a 1-form [itex]\omega=\sum_{i} f_i\,dx_i[itex] on R3,

[itex]d \omega=\sum_{i,j}\frac{\partial f_i}{\partial x_j} dx_j\wedge dx_i,[itex]

which restricted to the three-dimensional case [itex]\omega= u\,dx+v\,dy+w\,dz [itex] is

[itex]d \omega = \left(\frac{\partial v}{\partial x} - \frac{\partial u}{\partial y} \right) dx \wedge dy

+ \left(\frac{\partial w}{\partial y} - \frac{\partial v}{\partial z} \right) dy \wedge dz + \left(\frac{\partial u}{\partial z} - \frac{\partial w}{\partial x} \right) dz \wedge dx.[itex]

Therefore, for vector field V=[u,v,w] we have [itex]d \omega(U,W)=\langle\mbox{curl}\, V \times U,W\rangle [itex] where curl V denotes the curl of V, × is the vector product, and <•, •> is the scalar product.

(what are U and W here? this assertion needs clarification - Gauge 23:37, 7 Apr 2005 (UTC))

### Divergence

For a 2-form [itex] \omega = \sum_{i,j} h_{i,j}\,dx_i\,dx_j,[itex]

[itex]d \omega = \sum_{i,j,k} \frac{\partial h_{i,j}}{\partial x_k} dx_k \wedge dx_i \wedge dx_j.[itex]

For three dimensions, with [itex] \omega = p\,dy\wedge dz+q\,dz\wedge dx+r\,dx\wedge dy[itex] we get

[itex]d \omega = \left( \frac{\partial p}{\partial x} + \frac{\partial q}{\partial y} + \frac{\partial r}{\partial z} \right) dx \wedge dy \wedge dz = \mbox{div}V dx \wedge dy \wedge dz,[itex]

where V is a vector field defined by [itex] V = [p,q,r].[itex]

## Examples

For a 1-form [itex]\sigma = u\, dx + v\, dy[itex] on R2 we have

[itex]d \sigma = \left(\frac{\partial{v}}{\partial{x}} - \frac{\partial{u}}{\partial{y}}\right) dx \wedge dy[itex]

which is exactly the 2-form being integrated in Green's theorem.

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