where is a vector space over the field (or more generally, a module over a commutative ring), that is separately K-linear in each of its arguments. (The rest of this article, however, will only consider multilinear forms on finite-dimensional vector spaces.)
A multilinear k-form on over is called a (covariant) k-tensor, and the vector space of such forms is usually denoted or .
An important class of multilinear forms are the alternating multilinear forms, which have the additional property that
where is a permutation and denotes its sign (+1 if even, –1 if odd). As a consequence, alternating multilinear forms are antisymmetric with respect to swapping of any two arguments (i.e., and ):
With the additional hypothesis that the characteristic of the field is not 2, setting implies as a corollary that ; that is, the form has a value of 0 whenever two of its arguments are equal. Note, however, that some authors use this last condition as the defining property of alternating forms. This definition implies the property given at the beginning of the section, but as noted above, the converse implication holds only when .
An alternating multilinear k-form on over is called a multicovector of degree k or k-covector, and the vector space of such alternating forms, a subspace of , is generally denoted , or, using the notation for the isomorphic kth exterior power of (the dual space of ), . Note that linear functionals (multilinear 1-forms over ) are trivially alternating, so that , while, by convention, 0-forms are defined to be scalars: .
The determinant on matrices, viewed as an argument function of the column vectors, is an important example of an alternating multilinear form.
The tensor product of alternating multilinear forms is, in general, no longer alternating. However, by summing over all permutations of the tensor product, taking into account the parity of each term, the wedge product () of multicovectors can be defined, so that if and , then :
where the sum is taken over the set of all permutations over elements, . The wedge product is bilinear, associative, and anticommutative: if and then .
Given a basis for and dual basis for , the wedge products , with form a basis for . Hence, the dimensionality of for n-dimensional is .
Differential forms are mathematical objects constructed via tangent spaces and multilinear forms that behave, in many ways, like differentials in the classical sense. Though conceptually and computationally useful, differentials are founded on ill-defined notions of infinitesimal quantities developed early in the history of calculus. Differential forms provide a mathematically rigorous and precise framework to modernize this long-standing idea. Differential forms are especially useful in multivariable calculus (analysis) and differential geometry because they possess transformation properties that allow them be integrated on curves, surfaces, and their higher-dimensional analogues (differentiable manifolds). One far-reaching application is the modern statement of Stokes' theorem, a sweeping generalization of the fundamental theorem of calculus to higher dimensions.
The synopsis below is primarily based on Spivak (1965) and Tu (2011).
Definition and construction of differential 1-forms
To define differential forms on open subsets , we first need the notion of the tangent space of at , usually denoted or . The vector space can be defined most conveniently as the set of elements (, with fixed) with vector addition and scalar multiplication defined by and , respectively. Moreover, if is the standard basis for , then is the analogous standard basis for . In other words, each tangent space can simply be regarded as a copy of (a set of tangent vectors) based at the point . The collection (disjoint union) of tangent spaces of at all is known as the tangent bundle of and is usually denoted . While the definition given here provides a simple description of the tangent space of , there are other, more sophisticated constructions that are better suited for defining the tangent spaces of smooth manifolds in general (see the article on tangent spaces for details).
A differential k-form on is defined as a function that assigns to every a k-covector on the tangent space of at , usually denoted . In brief, a differential k-form is a k-covector field. The space of k-forms on is usually denoted ; thus if is a differential k-form, we write . By convention, a continuous function on is a differential 0-form: .
We first construct differential 1-forms from 0-forms and deduce some of their basic properties. To simplify the discussion below, we will only consider smooth differential forms constructed from smooth () functions. Let be a smooth function. We define the 1-form on for and by , where is the total derivative of at . (Recall that the total derivative is a linear transformation.) Of particular interest are the projection maps (also known as coordinate functions) , defined by , where is the ith standard coordinate of . The 1-forms are known as the basic 1-forms; they are conventionally denoted . If the standard coordinates of are , then application of the definition of yields , so that , where is the Kronecker delta. Thus, as the dual of the standard basis for , forms a basis for . As a consequence, if is a 1-form on , then can be written as for smooth functions . Furthermore, we can derive an expression for that coincides with the classical expression for a total differential:
[Comments onnotation: In this article, we follow the convention from tensor calculus and differential geometry in which multivectors and multicovectors are written with lower and upper indices, respectively. Since differential forms are multicovector fields, upper indices are employed to index them. The opposite rule applies to the components of multivectors and multicovectors, which instead are written with upper and lower indices, respectively. For instance, we represent the standard coordinates of vector as , so that in terms of the standard basis . In addition, superscripts appearing in the denominator of an expression (as in ) are treated as lower indices in this convention. When indices are applied and interpreted in this manner, the number of upper indices minus the number of lower indices in each term of an expression is conserved, both within the sum and across an equal sign, a feature that serves as a useful mnemonic device and helps pinpoint errors made during manual computation.]
The wedge product () and exterior differentiation () are two fundamental operations on differential forms. The wedge product of a k-form and an ℓ-form is a -form, while the exterior derivative of a k-form is a -form. Thus, both operations generate differential forms of higher degree from those of lower degree.
The wedge product of differential forms is a special case of the wedge product of multicovectors in general (see above). As is true in general for the wedge product, the wedge product of differential forms is bilinear, associative, and anticommutative.
More concretely, if and , then
Furthermore, for any set of indices ,
If , , and , then the indices of can be arranged in ascending order by a (finite) sequence of such swaps. Since , implies that . Finally, as a consequence of bilinearity, if and are the sums of several terms, their wedge product obeys distributivity with respect to each of these terms.
The collection of the wedge products of basic 1-forms constitutes a basis for the space of differential k-forms. Thus, any can be written in the form
where are smooth functions. With each set of indices placed in ascending order, (*) is said to be the standard presentation of .
In the previous section, the 1-form was defined by taking the exterior derivative of the 0-form (continuous function) . We now extend this by defining the exterior derivative operator for . If the standard presentation of k-form is given by (*), the -form is defined by
Integration of differential forms and Stokes' theorem for chains
To integrate a differential form over a parameterized domain, we first need to introduce the notion of the pullback of a differential form. Roughly speaking, when a differential form is integrated, applying the pullback transforms it in a way that correctly accounts for a change-of-coordinates.
Given a differentiable function and k-form , we call the pullback of by and define it as the k-form such that
for , where is the map .
If is an n-form on (i.e., ), we define its integral over the unit n-cell as the iterated Riemann integral of :
Next, we consider a domain of integration parameterized by a differentiable function , known as an n-cube. To define the integral of over , we "pull back" from to the unit n-cell:
To integrate over more general domains, we define an n-chain as the formal sum of n-cubes and set
An appropriate definition of the -chain, known as the boundary of , allows us to state the celebrated Stokes' theorem (Stokes–Cartan theorem) for chains in a subset of :
If is asmooth-form on an open set and is a smooth-chain in , then.
Using more sophisticated machinery (e.g., germs and derivations), the tangent space of any smooth manifold (not necessarily embedded in ) can be defined. Analogously, a differential form on a general smooth manifold is a map . Stokes' theorem can be further generalized to arbitrary smooth manifolds-with-boundary and even certain "rough" domains (see the article on Stokes' theorem for details).
^The Kronecker delta is usually denoted by and defined as . Here, the notation is used to conform to the tensor calculus convention on the use of upper and lower indices.
^The formal definition of the boundary of a chain is somewhat involved and is omitted here (see Spivak (1965), pp. 98-99 for a discussion). Intuitively, if maps to a square, then is a linear combination of functions that maps to its edges in a counterclockwise manner. It should be noted that the boundary of a chain is distinct from the notion of a boundary in point-set topology.