In mathematics, especially vector calculus and differential topology, a closed form is a differential form α whose exterior derivative is zero (dα = 0), and an exact form is a differential form that is the exterior derivative of another differential form β. Thus, an exact form is in the image of d, and a closed form is in the kernel of d.
For an exact form α, α = dβ for some differential form β of one-lesser degree than α. The form β is called a "potential form" or "primitive" for α. Since d^{2} = 0, β is not unique, but can be modified by the addition of the differential of a two-step-lower-order form.
Because d^{2} = 0, any exact form is automatically closed. The question of whether every closed form is exact depends on the topology of the domain of interest. On a contractible domain, every closed form is exact by the Poincaré lemma. More general questions of this kind on an arbitrary differentiable manifold are the subject of de Rham cohomology, that allows one to obtain purely topological information using differential methods.
Examples
The simplest example of a form which is closed but not exact is the 1-form "dθ" (quotes because it is not the derivative of a globally defined function), defined on the punctured plane $\backslash mathbf\{R\}^2\backslash setminus\backslash \{0\backslash \},$ which is locally given as the derivative of the argument - note that argument is locally but not globally defined, since a loop around the origin increases (or decreases, depending on direction) the argument by 2π, which corresponds to the integral:
- $\backslash oint\_\{S^1\}\; d\backslash theta\; =\; 2\backslash pi,$
and for general paths is known as the winding number. The differential of the argument is however globally defined (except at the origin), since differentiation only requires local data and different values of the argument differ by a constant, so the derivatives of different local definitions are equal; this line of thought is generalized in the notion of covering spaces.
Explicitly, the form is given as:
- $d\backslash theta\; =\; \backslash frac\{1\}\{x^2+y^2\}\backslash left(-y\backslash ,dx\; +\; x\backslash ,dy\backslash right),$
which is not defined at the origin. This can be computed from a formula for the argument, most simply via arctan(y/x) (y/x is the slope of the line passing through (x,y), and arctan converts slope to angle), recognizing 1/(x^{2}+y^{2}) as corresponding to the derivative of arctan, which is 1/(x^{2}+1) (these agree on the line y=1). While the differential is correctly computed by symbolically differentiating this expression, this formula is only strictly correct on the halfplane x>0, and properly one must use a correct formula for the argument.
This form generates the de Rham cohomology group $H^1\_\{dR\}(\backslash mathbf\{R\}^2\backslash setminus\backslash \{0\backslash \})\; \backslash cong\; \backslash mathbf\{R\},$ meaning that any closed form $\backslash omega$ is the sum of an exact form $df$ and a multiple of $d\backslash theta:$ $\backslash omega\; =\; df\; +\; k\backslash cdot\; d\backslash theta,$ where $\backslash textstyle\{k\; =\; \backslash frac\{1\}\{2\backslash pi\}\backslash oint\_\{S^1\}\; \backslash omega\}$ accounts for a non-trivial contour integral around the origin, which is the only obstruction to a closed form on the punctured plane (locally the derivative of a potential function) being the derivative of a globally defined function.
Examples in low dimensions
Differential forms in R^{2} and R^{3} were well known in the mathematical physics of the nineteenth century. In the plane, 0-forms are just functions, and 2-forms are functions times the basic area element dx∧dy, so that it is the 1-forms
- $\backslash alpha\; =\; f(x,y)\; \backslash ,\; dx\; +\; g(x,y)\; \backslash ,\; dy$
that are of real interest. The formula for the exterior derivative d here is
- $d\; \backslash alpha\; =\; (g\_x-f\_y)\; \backslash ,\; dx\backslash wedge\; dy\backslash ,$
where the subscripts denote partial derivatives. Therefore the condition for α to be closed is
- $f\_y=g\_x.\backslash ,$
In this case if h(x,y) is a function then
- $dh\; =\; h\_x\; \backslash ,\; dx\; +\; h\_y\; \backslash ,\; dy.\backslash ,$
The implication from 'exact' to 'closed' is then a consequence of the symmetry of second derivatives, with respect to x and y.
The gradient theorem asserts that a 1-form is exact if and only if the line integral of the form depends only on the endpoints of the curve, or equivalently,
if the integral around any smooth closed curve is zero.
Vector field analogies
On a Riemannian manifold, or more generally a pseudo-Riemannian manifold, k-forms correspond to k-vector fields (by duality via the metric), so there is a notion of a vector field corresponding to a closed or exact form.
In 3 dimensions, an exact vector field (thought of as a 1-form) is called a conservative vector field, meaning that it is the derivative (gradient) of a 0-form (function), called the scalar potential. A closed vector field (thought of as a 1-form) is one whose derivative (curl) vanishes, and is called an irrotational vector field.
Thinking of a vector field as a 2-form instead, a closed vector field is one whose derivative (divergence) vanishes, and is called an incompressible flow (sometimes solenoidal vector field).
Conservative and incompressible vector fields generalize to n-dimensions (gradient and divergence generalize to n dimensions); curl and hence irrotational does not generalize in this way.
Poincaré lemma
The Poincaré lemma states that if X is a contractible open subset of R^{n}, any smooth closed p-form α defined on X is exact, for any integer p > 0 (this has content only when p ≤ n).
Contractibility means that there is a homotopy F_{t} : X×[0,1] → X that continuously deforms X to a point. Thus every cycle c in X is the boundary of some "cone"; one may take the cone to be the image of c under the homotopy. A dual version of this gives the Poincaré lemma.
More specifically, we associate to X the cylinder X×[0,1]. Identify the top and bottom of the cylinder with the maps j_{1}(x) = (x, 1) and j_{0}(x) = (x, 0) respectively. On the differential forms, the induced maps j_{1}* and j_{0}* are related by a cochain homotopy K:
- $K\; d\; +\; d\; K\; =\; j\_1^*\; -\; j\_0\; ^*.$
Let Ω^{p}(X) denote the p-forms on X. The map K: Ω^{p + 1}( X×[0,1] ) → Ω^{p}(X) is the dual of the cylinder map and defined by
- $a(x,t)\; \backslash ,\; d\; x^\{p+1\}\; \backslash mapsto\; 0,\; \backslash ;\; a(x,t)\; \backslash ,\; dt\; \backslash ,\; dx^p\; \backslash mapsto\; \backslash left(\backslash int\_0\; ^1\; a(x,t)\; \backslash ,\; dt\backslash right)\; dx^p,$
where dx^{p} is a monomial p-form with no dt in it. So if F is a homotopy deforming X to a point Q, then
- $F\; \backslash circ\; j\_1\; =\; \backslash mathrm\{id\},\; \backslash ;\; F\; \backslash circ\; j\_0\; =\; Q.$
On forms,
- $j\_1\; ^*\; \backslash circ\; F^*\; =\; \backslash mathrm\{id\},\; \backslash ;\; j\_0^*\; \backslash circ\; F^*\; =\; 0.$
Inserting these two equations into the cochain homotopy equation proves the Poincaré lemma.
Formulation as cohomology
When the difference of two closed forms is an exact form, they are said to be cohomologous to each other. That is, if ζ and η are closed forms, and one can find some β such that
- $\backslash zeta\; -\; \backslash eta\; =\; d\backslash beta\backslash ,$
then one says that ζ and η are cohomologous to each other. Exact forms are sometimes said to be cohomologous to zero. The set of all forms cohomologous to a given form (and thus to each other) is called a de Rham cohomology class; the general study of such classes is known as cohomology. It makes no real sense to ask whether a 0-form (smooth function) is exact, since d increases degree by 1; but the clues from topology suggest that only the zero function should be called "exact". The cohomology classes are identified with locally constant functions.
A corollary of the Poincaré lemma is that de Rham cohomology is homotopy-invariant. Non-contractible spaces need not have trivial de Rham cohomology. For instance, on the circle S^{1}, parametrized by t in [0, 1], the closed 1-form dt is not exact.
Application in electrodynamics
In electrodynamics, the case of the magnetic field $\backslash vec\; B(\backslash mathbf\; r)$ produced by a stationary electrical current is important. There one deals with the vector potential $\backslash vec\; A(\backslash mathbf\; r\; )$ of this field. This case corresponds to k=2, and the defining region is the full $\backslash mathbb\; R^3\backslash ,.$ The current-density vector is $\backslash vec\; j\backslash ,.$ It corresponds to the current two-form
- $\backslash mathbf\; I:=j\_1(x,y,z)\; \backslash ,\; \{\backslash rm\; d\}x\_2\backslash wedge\; \{\backslash rm\; d\}x\_3+j\_2(x,y,z)\; \backslash ,\; \{\backslash rm\; d\}x\_3\backslash wedge\; \{\backslash rm\; d\}x\_1+j\_3(x,y,z)\; \backslash ,\; \{\backslash rm\; d\}x\_1\backslash wedge\; \{\backslash rm\; d\}x\_2.$
For the magnetic field $\backslash vec\; B$ one has analogous results: it corresponds to the induction two-form $\backslash Phi\_B:=B\_1\{\backslash rm\; d\}x\_2\backslash wedge\; \{\backslash rm\; d\}x\_3\; +\backslash cdots\; ,$ and can be derived from the vector potential $\backslash vec\; A$, or the corresponding one-form $\backslash mathbf\; A$,
- $\backslash vec\; B\; =\{\backslash rm\; curl\backslash ,\backslash ,\}\backslash vec\; A\; =\backslash left\backslash \{\; \backslash frac\{\backslash partial\; A\_3\}\{\backslash partial\; x\_2\}-\backslash frac\{\backslash partial\; A\_2\}\{\backslash partial\; x\_3\}\; ,\; \backslash frac\{\backslash partial\; A\_1\}\{\backslash partial\; x\_3\}-\backslash frac\{\backslash partial\; A\_3\}\{\backslash partial\; x\_1\}\; ,\backslash frac\{\backslash partial\; A\_2\}\{\backslash partial\; x\_1\}-\backslash frac\{\backslash partial\; A\_1\}\{\backslash partial\; x\_2\}\backslash right\backslash \},\backslash text\{\; or\; \}\backslash Phi\_B=\{\backslash rm\; d\}\backslash mathbf\; A.$
Thereby the vector potential $\backslash vec\; A$ corresponds to the potential one-form
- $\backslash mathbf\; A:=A\_1\; \backslash ,\; \{\backslash rm\; d\}x\_1+A\_2\; \backslash ,\; \{\backslash rm\; d\}x\_2+A\_3\; \backslash ,\; \{\backslash rm\; d\}x\_3.$
The closedness of the magnetic-induction two-form corresponds to the property of the magnetic field that it is source-free: $\{\backslash rm\; div\backslash ,\backslash ,\}\backslash vec\; B\; \backslash equiv\; 0,$ i.e. there are no magnetic monopoles.
In a special gauge, $\{\backslash rm\; div\backslash ,\backslash ,\}\backslash vec\; A\backslash stackrel\{!\}\{=\}0$, this implies for i = 1, 2, 3
- $A\_i(\backslash vec\; r)$
=\int \frac{\mu_0 j_i(\vec r^{\,'})\,\, dx_1'dx_2'dx_3'}{4\pi |\vec r -\vec r^{\,'}|}\,.
(Here $\backslash mu\_0$ is a constant, the magnetic vacuum permeability.)
This equation is remarkable, because it corresponds completely to a well-known formula for the electrical field $\backslash vec\; E$, namely for the electrostatic Coulomb potential $\backslash ,\backslash phi\; (x\_1,x\_2,\; x\_3)$ of a charge density $\backslash rho\; (x\_1,x\_2,x\_3)$. At this place one can already guess that
- $\backslash vec\; E$ and $\backslash vec\; B\; ,$
- $\backslash rho$ and $\backslash vec\; j\; ,$
- $\backslash ,\backslash phi$ and $\backslash vec\; A$
can be unified to quantities with six rsp. four nontrivial components, which is the basis of the relativistic invariance of the Maxwell equations.
If the condition of stationarity is left, on the l.h.s. of the above-mentioned equation one must add, in the equations for $A\_i\backslash ,,$ to the three space coordinates, as a fourth variable also the time t, whereas on the r.h.s., in $j\_i\text{'}\; \backslash ,,$ the so-called "retarded time", $t\text{'}:=t-\backslash frac\{|\backslash vec\; r\; -\backslash vec\; r^\{\backslash ,\text{'}\}|\}\{c\}\backslash ,,$ must be used, i.e. it is added to the argument of the current-density. Finally, as before, one integrates over the three primed space coordinates. (As usual
c is the vacuum velocity of light.)
References
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.