# Exterior derivative

On a differentiable manifold, the **exterior derivative** extends the concept of the differential of a function to differential forms of higher degree. The exterior derivative was first described in its current form by Élie Cartan in 1899. It allows for a natural, metric-independent generalization of Stokes' theorem, Gauss's theorem, and Green's theorem from vector calculus.

If a differential *k*-form is thought of as measuring the flux through an infinitesimal *k*-parallelotope at each point of the manifold, then its exterior derivative can be thought of as measuring the net flux through the boundary of a (*k* + 1)-parallelotope at each point.

The exterior derivative of a differential form of degree *k* (also differential *k*-form, or just *k*-form for brevity here) is a differential form of degree *k* + 1.

If *f* is a smooth function (a 0-form), then the exterior derivative of *f* is the differential of *f* . That is, *df* is the unique 1-form such that for every smooth vector field *X*, *df* (*X*) = *d*_{X} *f* , where *d*_{X} *f* is the directional derivative of *f* in the direction of *X*.

The exterior product of differential forms (denoted with the same symbol ∧) is defined as their pointwise exterior product.

There are a variety of equivalent definitions of the exterior derivative of a general *k*-form.

The exterior derivative is defined to be the unique ℝ-linear mapping from *k*-forms to (*k* + 1)-forms that has the following properties:

The second defining property holds in more generality: *d*(*dα*) = 0 for any *k*-form α; more succinctly, *d*^{2} = 0. The third defining property implies as a special case that if *f* is a function and α a is *k*-form, then *d*( *fα*) = *d*( *f* ∧ *α*) = *df* ∧ *α* + *f* ∧ *dα* because a function is a 0-form, and scalar multiplication and the exterior product are equivalent when one of the arguments is a scalar.^{[citation needed]}

Alternatively, one can work entirely in a local coordinate system (*x*^{1}, ..., *x*^{n}). The coordinate differentials *dx*^{1}, ..., *dx*^{n} form a basis of the space of one-forms, each associated with a coordinate. Given a multi-index *I* = (*i*_{1}, ..., *i*_{k}) with 1 ≤ *i*_{p} ≤ *n* for 1 ≤ *p* ≤ *k* (and denoting *dx*^{i1} ∧ ... ∧ *dx*^{ik} with an abuse of notation *dx*^{I}), the exterior derivative of a (simple) *k*-form

(using the Einstein summation convention). The definition of the exterior derivative is extended linearly to a general *k*-form

The definition of the exterior derivative in local coordinates follows from the preceding definition in terms of axioms. Indeed, with the *k*-form *φ* as defined above,

Here, we have interpreted *g* as a 0-form, and then applied the properties of the exterior derivative.

In particular, for a 1-form *ω*, the components of *dω* in local coordinates are

Alternatively, an explicit formula can be given^{[citation needed]} for the exterior derivative of a *k*-form *ω*, when paired with *k* + 1 arbitrary smooth vector fields *V*_{0}, *V*_{1}, ..., *V*_{k}:

where [*V _{i}*,

*V*] denotes the Lie bracket

_{j}^{[further explanation needed]}and a hat denotes the omission of that element:

In particular, when *ω* is a 1-form we have that *dω*(*X*, *Y*) = *d*_{X}(*ω*(*Y*)) − *d*_{Y}(*ω*(*X*)) − *ω*([*X*, *Y*]).

**Note:** With the conventions of e.g., Kobayashi–Nomizu and Helgason the formula differs by a factor of 1/*k* + 1:

**Example 1.** Consider *σ* = *u* *dx*^{1} ∧ *dx*^{2} over a 1-form basis *dx*^{1}, ..., *dx*^{n} for a scalar field *u*. The exterior derivative is:

The last formula, where summation starts at *i* = 3, follows easily from the properties of the exterior product. Namely, *dx*^{i} ∧ *dx*^{i} = 0.

**Example 2.** Let *σ* = *u* *dx* + *v* *dy* be a 1-form defined over ℝ^{2}. By applying the above formula to each term (consider *x*^{1} = *x* and *x*^{2} = *y*) we have the following sum,

If *M* is a compact smooth orientable *n*-dimensional manifold with boundary, and *ω* is an (*n* − 1)-form on *M*, then the generalized form of Stokes' theorem states that:

Intuitively, if one thinks of *M* as being divided into infinitesimal regions, and one adds the flux through the boundaries of all the regions, the interior boundaries all cancel out, leaving the total flux through the boundary of *M*.

A *k*-form *ω* is called *closed* if *dω* = 0; closed forms are the kernel of *d*. *ω* is called *exact* if *ω* = *dα* for some (*k* − 1)-form *α*; exact forms are the image of *d*. Because *d*^{2} = 0, every exact form is closed. The Poincaré lemma states that in a contractible region, the converse is true.

Because the exterior derivative *d* has the property that *d*^{2} = 0, it can be used as the differential (coboundary) to define de Rham cohomology on a manifold. The *k*-th de Rham cohomology (group) is the vector space of closed *k*-forms modulo the exact *k*-forms; as noted in the previous section, the Poincaré lemma states that these vector spaces are trivial for a contractible region, for *k* > 0. For smooth manifolds, integration of forms gives a natural homomorphism from the de Rham cohomology to the singular cohomology over ℝ. The theorem of de Rham shows that this map is actually an isomorphism, a far-reaching generalization of the Poincaré lemma. As suggested by the generalized Stokes' theorem, the exterior derivative is the "dual" of the boundary map on singular simplices.

The exterior derivative is natural in the technical sense: if *f* : *M* → *N* is a smooth map and Ω^{k} is the contravariant smooth functor that assigns to each manifold the space of *k*-forms on the manifold, then the following diagram commutes

so *d*( *f*^{∗}*ω*) = *f*^{∗}*dω*, where *f*^{∗} denotes the pullback of *f* . This follows from that *f*^{∗}*ω*(·), by definition, is *ω*( *f*_{∗}(·)), *f*_{∗} being the pushforward of *f* . Thus *d* is a natural transformation from Ω^{k} to Ω^{k+1}.

Most vector calculus operators are special cases of, or have close relationships to, the notion of exterior differentiation.

A smooth function *f* : *M* → ℝ on a real differentiable manifold *M* is a 0-form. The exterior derivative of this 0-form is the 1-form *df*.

When an inner product ⟨·,·⟩ is defined, the gradient ∇*f* of a function *f* is defined as the unique vector in *V* such that its inner product with any element of *V* is the directional derivative of *f* along the vector, that is such that

where ♯ denotes the musical isomorphism ♯ : *V*^{∗} → *V* mentioned earlier that is induced by the inner product.

The 1-form *df* is a section of the cotangent bundle, that gives a local linear approximation to *f* in the cotangent space at each point.

A vector field *V* = (*v*_{1}, *v*_{2}, ..., *v _{n}*) on ℝ

^{n}has a corresponding (

*n*− 1)-form

(For instance, when *n* = 3, i.e. in three-dimensional space, the 2-form *ω _{V}* is locally the scalar triple product with

*V*.) The integral of

*ω*over a hypersurface is the flux of

_{V}*V*over that hypersurface.

Locally, *η _{V}* is the dot product with

*V*. The integral of

*η*along a path is the work done against −

_{V}*V*along that path.

When *n* = 3, in three-dimensional space, the exterior derivative of the 1-form *η _{V}* is the 2-form

The standard vector calculus operators can be generalized for any pseudo-Riemannian manifold, and written in coordinate-free notation as follows:

where ⋆ is the Hodge star operator, ♭ and ♯ are the musical isomorphisms, *f* is a scalar field and *F* is a vector field.

Note that the expression for curl requires ♯ to act on ⋆*d*(*F*^{♭}), which is a form of degree *n* − 2. A natural generalization of ♯ to *k*-forms of arbitrary degree allows this expression to make sense for any *n*.