Differential form
In the mathematical fields of differential geometry and tensor calculus, differential forms are an approach to multivariable calculus that is independent of coordinates. Differential forms provide a unified approach to defining integrands over curves, surfaces, volumes, and higherdimensional manifolds. The modern notion of differential forms was pioneered by Élie Cartan. It has many applications, especially in geometry, topology and physics.
For instance, the expression f(x) dx from onevariable calculus is an example of a 1form, and can be integrated over an interval [a, b] in the domain of f:
and similarly the expression f(x, y, z) dx ∧ dy + g(x, y, z) dx ∧ dz + h(x, y, z) dy ∧ dz is a 2form that has a surface integral over an oriented surface S:
Likewise, a 3form f(x, y, z) dx ∧ dy ∧ dz represents a volume element that can be integrated over a region of space.
The algebra of differential forms is organized in a way that naturally reflects the orientation of the domain of integration. There is an operation d on differential forms known as the exterior derivative that, when acting on a kform, produces a (k + 1)form. This operation extends the differential of a function, and the divergence and the curl of a vector field in an appropriate sense that makes the fundamental theorem of calculus, the divergence theorem, Green's theorem, and Stokes' theorem special cases of the same general result, known in this context also as the general Stokes' theorem. In a deeper way, this theorem relates the topology of the domain of integration to the structure of the differential forms themselves; the precise connection is known as de Rham's theorem.
The general setting for the study of differential forms is on a differentiable manifold. Differential 1forms are naturally dual to vector fields on a manifold, and the pairing between vector fields and 1forms is extended to arbitrary differential forms by the interior product. The algebra of differential forms along with the exterior derivative defined on it is preserved by the pullback under smooth functions between two manifolds. This feature allows geometrically invariant information to be moved from one space to another via the pullback, provided the information is expressed in terms of differential forms. As an example, the change of variables formula for integration becomes a simple statement that an integral is preserved under pullback.
History
Differential forms are part of the field of differential geometry, influenced by linear algebra. Although the notion of a differential is quite old, the initial attempt at an algebraic organization of differential forms is usually credited to Élie Cartan with reference to his 1899 paper.^{[1]}
Concept
Differential forms provide an approach to multivariable calculus that is independent of coordinates.
Integration
A differential kform can be integrated over a manifold of dimension k. A differential oneform can be thought of as measuring an infinitesimal (oriented) length, or onedimensional density. A differential twoform can be thought of as measuring an infinitesimal (oriented) area, or twodimensional density. And so on.
Integration of differential forms is only welldefined on oriented manifolds. An example of a one dimensional manifold is an interval [a, b], and intervals can be given an orientation: they are positively oriented if a < b, and negatively oriented otherwise. If a < b then the integral of the differential oneform f(x) dx over the interval [a, b] (with its natural positive orientation) is
which is the negative of the integral of the same differential form over the same interval, when equipped with the opposite orientation. That is:
This gives a geometrical context to the conventions for onedimensional integrals, that the sign changes when the orientation of the interval is reversed. A standard explanation of this in onevariable integration theory is that, when the limits of integration are in the opposite order (b < a), the increment dx is negative. The integrals are negatives of one another because the oriented lengths "dx" have opposite directions.
More generally, an mform is an oriented density that can be integrated over an mdimensional oriented manifold. (For example, a 1form can be integrated over an oriented curve, a 2form can be integrated over an oriented surface, etc.) If M is an oriented mdimensional manifold, and M′ is the same manifold with opposed orientation and ω is an mform, then one has:
These conventions correspond to interpreting the integrand as a differential form, integrated over a chain. In measure theory, by contrast, one interprets the integrand as a function f with respect to a measure μ and integrates over a subset A, without any notion of orientation; one writes to indicate integration over a subset A. This is a minor distinction in one dimension, but becomes subtler on higherdimensional manifolds; see below for details.
Making the notion of an oriented density precise, and thus of a differential form, involves the exterior algebra. The basic 1forms are the differentials of the coordinates: dx^{1}, ..., dx^{n}. Each of these represents a covector that measures a small displacement in the corresponding coordinate direction. A general 1form is a linear combination of these differentials
where the are functions of the coordinates. A differential 1form is integrated along an oriented curve as a line integral.
The basic twoforms are expressions dx^{i} ∧ dx^{j}, where i < j. This represents an infinitesimal oriented square parallel to the x^{i}–x^{j}plane. A general twoform is a linear combination of these, and it is integrated just like a surface integral.
A fundamental operation defined on differential forms is the wedge product (the symbol is the wedge ∧). This is similar to the cross product from vector calculus, in that it is an alternating product. For instance,
because the square whose first side is dx^{1} and second side is dx^{2} is to be regarded as having the opposite orientation as the square whose first side is dx^{2} and whose second side is dx^{1}. The wedge product allows higher dimensional differential forms to be built out of lowerdimensional ones, in much the same way that the cross product in vector calculus allows one to compute the area vector of a parallelogram from vectors pointing up the two sides.
In addition to the wedge product, there is also the exterior derivative operator d. Like the differential of a function, the exterior derivative gives a way of quantifying a differential form's sensitivity to change. In R^{n}, if ω = f dx^{a} is a kform, then dω is defined by
with extension to general kforms occurring linearly.
This more general approach allows for a more natural coordinatefree approach to integration on manifolds. It also allows for a natural generalization of the fundamental theorem of calculus (see § Stokes' theorem).
Differential calculus
Let U be an open set in R^{n}. A differential 0form ("zeroform") is defined to be a smooth function f on U. If v is any vector in R^{n}, then f has a directional derivative ∂_{v} f, which is another function on U whose value at a point p ∈ U is the rate of change (at p) of f in the v direction:
(This notion can be extended to the case that v is a vector field on U by evaluating v at the point p in the definition.)
In particular, if v = e_{j} is the jth coordinate vector then ∂_{v}f is the partial derivative of f with respect to the jth coordinate function, i.e., ∂f / ∂x^{j}, where x^{1}, x^{2}, ..., x^{n} are the coordinate functions on U. By their very definition, partial derivatives depend upon the choice of coordinates: if new coordinates y^{1}, y^{2}, ..., y^{n} are introduced, then
The first idea leading to differential forms is the observation that ∂_{v} f (p) is a linear function of v:
for any vectors v, w and any real number c. This linear map from R^{n} to R is denoted df_{p} and called the derivative of f at p. Thus df_{p}(v) = ∂_{v} f (p). The object df can be viewed as a function on U, whose value at p is not a real number, but the linear map df_{p}. This is just the usual Fréchet derivative – an example of a differential 1form.
Since any vector v is a linear combination ∑ v^{j}e_{j} of its components, df is uniquely determined by df_{p}(e_{j}) for each j and each p ∈ U, which are just the partial derivatives of f on U. Thus df provides a way of encoding the partial derivatives of f. It can be decoded by noticing that the coordinates x^{1}, x^{2}, ..., x^{n} are themselves functions on U, and so define differential 1forms dx^{1}, dx^{2}, ..., dx^{n}. Let f = x^{i}. Since ∂x^{i} / ∂x^{j} = δ^{j}_{i}, the Kronecker delta function, it follows that

(*)
The meaning of this expression is given by evaluating both sides at an arbitrary point p: on the right hand side, the sum is defined "pointwise", so that
Applying both sides to e_{j}, the result on each side is the jth partial derivative of f at p. Since p and j were arbitrary, this proves the formula (*).
More generally, for any smooth functions g_{i} and h_{i} on U, we define the differential 1form α = ∑_{i} g_{i} dh_{i} pointwise by
for each p ∈ U. Any differential 1form arises this way, and by using (*) it follows that any differential 1form α on U may be expressed in coordinates as
for some smooth functions f_{i} on U.
The second idea leading to differential forms arises from the following question: given a differential 1form α on U, when does there exist a function f on U such that α = df? The above expansion reduces this question to the search for a function f whose partial derivatives ∂f / ∂x^{i} are equal to n given functions f_{i}. For n > 1, such a function does not always exist: any smooth function f satisfies
so it will be impossible to find such an f unless
for all i and j.
The skewsymmetry of the left hand side in i and j suggests introducing an antisymmetric product ∧ on differential 1forms, the wedge product, so that these equations can be combined into a single condition
where ∧ is defined so that:
This is an example of a differential 2form. This 2form is called the exterior derivative dα of α = ∑n
j=1 f_{j} dx^{j}. It is given by
To summarize: dα = 0 is a necessary condition for the existence of a function f with α = df.
Differential 0forms, 1forms, and 2forms are special cases of differential forms. For each k, there is a space of differential kforms, which can be expressed in terms of the coordinates as
for a collection of functions f_{i1i2⋅⋅⋅ik}. (As assumed below, one can restrict the sum to the case i_{1} < i_{2} < ... < i_{k−1} < i_{k}.)
Differential forms can be multiplied together using the wedge product, and for any differential kform α, there is a differential (k + 1)form dα called the exterior derivative of α.
Differential forms, the wedge product and the exterior derivative are independent of a choice of coordinates. Consequently, they may be defined on any smooth manifold M. One way to do this is cover M with coordinate charts and define a differential kform on M to be a family of differential kforms on each chart which agree on the overlaps. However, there are more intrinsic definitions which make the independence of coordinates manifest.
Intrinsic definitions
Let M be a smooth manifold. A smooth differential form of degree k is a smooth section of the kth exterior power of the cotangent bundle of M. At any point p ∈ M, a kform β is an element
where T_{p}M is the tangent space to M at p. This space is naturally isomorphic to the dual of the kth exterior power of the tangent bundle of M, so β is also a linear functional
By the universal property of exterior powers, this is equivalently an alternating multilinear map
The exterior algebra may be embedded in the tensor algebra by means of the alternation map. More precisely, there is a map of vector bundles
defined at each point p by
where S_{k} is the symmetric group on k elements. This map exhibits β as a totally antisymmetric covariant tensor field of rank k. The differential forms on M are in onetoone correspondence with such tensor fields.
The set of all differential kforms on a manifold M is a vector space, often denoted Ω^{k}(M).
For example, a differential 1form α assigns to each point p ∈ M a linear functional α_{p} on T_{p}M. In the presence of an inner product on T_{p}M (induced by a Riemannian metric on M), α_{p} may be represented as the inner product with a tangent vector X_{p}. Differential 1forms are sometimes called covariant vector fields, covector fields, or "dual vector fields", particularly within physics.
Operations
As well as the addition and multiplication by scalar operations which arise from the vector space structure, there are several other standard operations defined on differential forms. The most important operations are the wedge product of two differential forms, the exterior derivative of a single differential form, the interior product of a differential form and a vector field, the Lie derivative of a differential form with respect to a vector field and the covariant derivative of a differential form with respect to a vector field on a manifold with a defined connection.
Wedge product
The wedge product of a kform α and an lform β is a (k + l)form denoted α ∧ β. At each point p of the manifold M, the forms α and β are elements of an exterior power of the tangent space at p. When the exterior algebra is viewed as a quotient of the tensor algebra, the wedge product corresponds to the tensor product.
The antisymmetry inherent in the exterior algebra means that when α ∧ β is viewed as a multilinear functional, it is alternating. However, when the exterior algebra is viewed as a subspace of the tensor algebra, the tensor product α ⊗ β is not alternating. There is an explicit formula which describes the wedge product in this situation. The wedge product is
This description is useful for explicit computations. For example, if k = l = 1, then α ∧ β is the 2form whose value at a point p is the alternating bilinear form defined by
for v, w ∈ T_{p}M.
The wedge product is bilinear: for instance, if α, β, and γ are any differential forms, then
It is skew commutative (also known as graded commutative), meaning that it satisfies a variant of anticommutativity that depends on the degrees of the forms: if α is a kform and β is an lform, then
Riemannian manifold
On a Riemannian manifold, or more generally a pseudoRiemannian manifold, vector and covector fields can be identified (the metric is a fiberwise isomorphism of the tangent space and the cotangent space), and additional operations can thus be defined, such as the Hodge star operator and codifferential (degree −1), which is adjoint to the exterior differential d.
Vector field structures
On a pseudoRiemannian manifold, 1forms can be identified with vector fields; vector fields have additional distinct algebraic structures, which are listed here for context and to avoid confusion.
Firstly, each (co)tangent space generates a Clifford algebra, where the product of a (co)vector with itself is given by the value of a quadratic form – in this case, the natural one induced by the metric. This algebra is distinct from the exterior algebra of differential forms, which can be viewed as a Clifford algebra where the quadratic form vanishes (since the exterior product of any vector with itself is zero). Clifford algebras are thus nonanticommutative ("quantum") deformations of the exterior algebra. They are studied in geometric algebra.
Another alternative is to consider vector fields as derivations, and consider the (noncommutative) algebra of differential operators they generate, which is the Weyl algebra, and is a noncommutative ("quantum") deformation of the symmetric algebra in the vector fields.
Exterior differential complex
One important property of the exterior derivative is that d^{2} = 0. This means that the exterior derivative defines a cochain complex:
By the Poincaré lemma, this complex is locally exact except at Ω^{0}(M). Its cohomology is the de Rham cohomology of M.
Pullback
One of the main reasons the cotangent bundle rather than the tangent bundle is used in the construction of the exterior complex is that differential forms are capable of being pulled back by smooth maps, while vector fields cannot be pushed forward by smooth maps unless the map is, say, a diffeomorphism. The existence of pullback homomorphisms in de Rham cohomology depends on the pullback of differential forms.
Differential forms can be moved from one manifold to another using a smooth map. If f : M → N is smooth and ω is a smooth kform on N, then there is a differential form f^{∗}ω on M, called the pullback of ω, which captures the behavior of ω as seen relative to f.
To define the pullback, recall that the differential of f is a map f_{∗} : TM → TN. Fix a differential kform ω on N. For a point p of M and tangent vectors v_{1}, ..., v_{k} to M at p, the pullback of ω is defined by the formula
More abstractly, if ω is viewed as a section of the cotangent bundle T^{∗}N of N, then f^{∗}ω is the section of T^{∗}M defined as the composite map
Pullback respects all of the basic operations on forms:
The pullback of a form can also be written in coordinates. Assume that x_{1}, ..., x_{m} are coordinates on M, that y_{1}, ..., y_{n} are coordinates on N, and that these coordinate systems are related by the formulas y_{i} = f_{i}(x_{1}, ..., x_{m}) for all i. Then, locally on N, ω can be written as
where, for each choice of i_{1}, ..., i_{k}, ω_{i1⋅⋅⋅ik} is a realvalued function of y_{1}, ..., y_{n}. Using the linearity of pullback and its compatibility with wedge product, the pullback of ω has the formula
Each exterior derivative df_{i} can be expanded in terms of dx_{1}, ..., dx_{m}. The resulting kform can be written using Jacobian matrices:
Here, denotes the determinant of the matrix whose entries are , .
Integration
A differential kform can be integrated over an oriented kdimensional manifold. More generally, it can be integrated over kdimensional chains. If k = 0, this is just evaluation of a function at points. Other values of k = 1, 2, 3, ... correspond to line integrals, surface integrals, volume integrals etc. Simply, a chain parametrizes a domain of integration as a collection of cells (images of cubes or other domains D) that are patched together; to integrate, one pulls back the form on each cell of the chain to a form on the cube (or other domain) and integrates there, which is just integration of a function on R^{k}, as the pulled back form is simply a multiple of the volume form du^{1} ∧ ... ∧ du^{k}. For example, given a path γ(t) : [0, 1] → R^{2}, integrating a form on the path is simply pulling back the form to a function on [0, 1] (properly, to a form f(t) dt) and integrating the function on the interval.
Formal details
Let
be a differential form and S a differentiable kmanifold over which we wish to integrate, where S has the parameterization
for u in the parameter domain D. Then (Rudin 1976) defines the integral of the differential form over S as
where the integral on the righthand side is the standard Riemann (or Lebesgue) integral over D, and
is the determinant of the Jacobian. The Jacobian exists because S is differentiable.
More generally, a kform can be integrated over a pdimensional submanifold, for p ≤ k, to obtain a (k − p)form. This comes up, for example, in defining the pushforward of a differential form by a smooth map f : M → N by attempting to integrate over the fibers of f.
Stokes' theorem
The fundamental relationship between the exterior derivative and integration is given by the Stokes' theorem: If ω is an (n − 1)form with compact support on M and ∂M denotes the boundary of M with its induced orientation, then
A key consequence of this is that "the integral of a closed form over homologous chains is equal": If ω is a closed kform and M and N are kchains that are homologous (such that M − N is the boundary of a (k + 1)chain W), then , since the difference is the integral .
For example, if ω = df is the derivative of a potential function on the plane or R^{n}, then the integral of ω over a path from a to b does not depend on the choice of path (the integral is f(b) − f(a)), since different paths with given endpoints are homotopic, hence homologous (a weaker condition). This case is called the gradient theorem, and generalizes the fundamental theorem of calculus). This path independence is very useful in contour integration.
This theorem also underlies the duality between de Rham cohomology and the homology of chains.
Relation with measures
On a general differentiable manifold (without additional structure), differential forms cannot be integrated over subsets of the manifold; this distinction is key to the distinction between differential forms, which are integrated over chains, and measures, which are integrated over subsets. The simplest example is attempting to integrate the 1form dx over the interval [0, 1]. Assuming the usual distance (and thus measure) on the real line, this integral is either 1 or −1, depending on orientation: , while . By contrast, the integral of the measure  dx  on the interval is unambiguously 1 (formally, the integral of the constant function 1 with respect to this measure is 1). Similarly, under a change of coordinates a differential nform changes by the Jacobian determinant J, while a measure changes by the absolute value of the Jacobian determinant,  J , which further reflects the issue of orientation. For example, under the map x ↦ −x on the line, the differential form dx pulls back to −dx; orientation has reversed; while the Lebesgue measure, which here we denote  dx , pulls back to  dx ; it does not change.
In the presence of the additional data of an orientation, it is possible to integrate nforms (topdimensional forms) over the entire manifold or over compact subsets; integration over the entire manifold corresponds to integrating the form over the fundamental class of the manifold, [M]. Formally, in the presence of an orientation, one may identify nforms with densities on a manifold; densities in turn define a measure, and thus can be integrated (Folland 1999, Section 11.4, pp. 361–362).
On an orientable but not oriented manifold, there are two choices of orientation; either choice allows one to integrate nforms over compact subsets, with the two choices differing by a sign. On nonorientable manifold, nforms and densities cannot be identified —notably, any topdimensional form must vanish somewhere (there are no volume forms on nonorientable manifolds), but there are nowherevanishing densities— thus while one can integrate densities over compact subsets, one cannot integrate nforms. One can instead identify densities with topdimensional pseudoforms.
There is in general no meaningful way to integrate kforms over subsets for k < n because there is no consistent way to orient kdimensional subsets; geometrically, a kdimensional subset can be turned around in place, reversing any orientation but yielding the same subset. Compare the Gram determinant of a set of k vectors in an ndimensional space, which, unlike the determinant of n vectors, is always positive, corresponding to a squared number.
On a Riemannian manifold, one may define a kdimensional Hausdorff measure for any k (integer or real), which may be integrated over kdimensional subsets of the manifold. A function times this Hausdorff measure can then be integrated over kdimensional subsets, providing a measuretheoretic analog to integration of kforms. The ndimensional Hausdorff measure yields a density, as above.
Applications in physics
Differential forms arise in some important physical contexts. For example, in Maxwell's theory of electromagnetism, the Faraday 2form, or electromagnetic field strength, is
where the f_{ab} are formed from the electromagnetic fields and ; e.g., f_{12} = E_{z}/c, f_{23} = −B_{z}, or equivalent definitions.
This form is a special case of the curvature form on the U(1) principal fiber bundle on which both electromagnetism and general gauge theories may be described. The connection form for the principal bundle is the vector potential, typically denoted by A, when represented in some gauge. One then has
The current 3form is
where j^{a} are the four components of the current density. (Here it is a matter of convention to write F_{ab} instead of f_{ab}, i.e. to use capital letters, and to write J^{a} instead of j^{a}. However, the vector rsp. tensor components and the abovementioned forms have different physical dimensions. Moreover, one should remember that by decision of an international commission of the IUPAP, the magnetic polarization vector is called since several decades, and by some publishers J, i.e. the same name is used for different quantities.)
Using the abovementioned definitions, Maxwell's equations can be written very compactly in geometrized units as
where denotes the Hodge star operator. Similar considerations describe the geometry of gauge theories in general.
The 2form , which is dual to the Faraday form, is also called Maxwell 2form.
Electromagnetism is an example of a U(1) gauge theory. Here the Lie group is U(1), the onedimensional unitary group, which is in particular abelian. There are gauge theories, such as Yang–Mills theory, in which the Lie group is not abelian. In that case, one gets relations which are similar to those described here. The analog of the field F in such theories is the curvature form of the connection, which is represented in a gauge by a Lie algebravalued oneform A. The Yang–Mills field F is then defined by
In the abelian case, such as electromagnetism, A ∧ A = 0, but this does not hold in general. Likewise the field equations are modified by additional terms involving wedge products of A and F, owing to the structure equations of the gauge group.
Applications in geometric measure theory
Numerous minimality results for complex analytic manifolds are based on the Wirtinger inequality for 2forms. A succinct proof may be found in Herbert Federer's classic text Geometric Measure Theory. The Wirtinger inequality is also a key ingredient in Gromov's inequality for complex projective space in systolic geometry.
See also
 Closed and exact differential forms
 Complex differential form
 Vectorvalued differential form
 Equivariant differential form
References
 ↑ Cartan, Élie (1899), "Sur certaines expressions différentielles et le problème de Pfaff", Annales scientifiques de l'École Normale Supérieure: 239–332
 Bachman, David (2006), A Geometric Approach to Differential Forms, Birkhäuser, ISBN 9780817644994
 Bachman, David (2003), A Geometric Approach to Differential Forms, arXiv:math/0306194v1
 Cartan, Henri (2006), Differential Forms, Dover, ISBN 0486450104—Translation of Formes différentielles (1967)
 Edwards, Harold M. (1994), Advanced Calculus; A Differential Forms Approach, Boston, Basel, Berlin: Birkhäuser, ISBN 9780817684112
 Folland, Gerald B. (1999), Real Analysis: Modern Techniques and Their Applications (Second ed.), ISBN 9780471317166, provides a brief discussion of integration on manifolds from the point of view of measure theory in the last section.
 Flanders, Harley (1989), Differential forms with applications to the physical sciences, Mineola, New York: Dover Publications, ISBN 0486661695
 Fleming, Wendell H. (1965), "Chapter 6: Exterior algebra and differential calculus", Functions of Several Variables, AddisonWesley, pp. 205–238. This textbook in multivariate calculus introduces the exterior algebra of differential forms at the college calculus level.
 Morita, Shigeyuki (2001), Geometry of Differential Forms, AMS, ISBN 0821810456
 Rudin, Walter (1976), Principles of Mathematical Analysis, New York: McGrawHill, ISBN 007054235X
 Spivak, Michael (1965), Calculus on Manifolds, Menlo Park, California: W. A. Benjamin, ISBN 0805390219, standard introductory text
 Tu, Loring W. (2008), An Introduction to Manifolds, Springer, ISBN 9780387480985
 Zorich, Vladimir A. (2004), Mathematical Analysis II, Springer, ISBN 3540406336
External links
 Sjamaar, Reyer (2006), Manifolds and differential forms lecture notes, a course taught at Cornell University.
 Bachman, David (2003), A Geometric Approach to Differential Forms, arXiv:math/0306194, an undergraduate text.
 Jones, Frank, Integration on manifolds (PDF)