Integrals of differential forms
A differential form is a mathematical concept in the fields of multivariable calculus, differential topology and tensors. The modern notation for the differential form, as well as the idea of the differential forms as being the wedge products of exterior derivatives forming an exterior algebra, was introduced by Élie Cartan.
We initially work in an open set in Rn. A 0-form is defined to be a smooth function f. When we integrate a function f over an m-dimensional subspace S of Rn, we write it as
(The superscripts are indices, not exponents.) We can consider dx1 through dxn to be formal objects themselves, rather than tags appended to make integrals look like Riemann sums. Alternatively, we can view them as covectors, and thus a measure of “density” (hence integrable in a general sense). We call the dx1, …,dxn basic 1-forms.
We define the wedge product, “∧”, a bilinear “multiplication” operator on these elements, with the alternating property that
for all indices a. Note that alternation along with linearity and associativity implies dxb∧dxa = −dxa∧dxb. This also ensures that the result of the wedge product has an orientation.
We define the set of all these products to be basic 2-forms, and similarly we define the set of products of the form dxa∧dxb∧dxc to be basic 3-forms. A general k-form is then a weighted sum of basic k-forms, where the weights are the smooth functions f. Together these form a vector space with basic k-forms as the basis vectors, and 0-forms (smooth functions) as the field of scalars. The wedge product then extends to k-forms in the natural way. Over Rn at most n covectors can be linearly independent, thus a k-form with k > n will always be zero, by the alternating property.
In addition to the wedge product, there is also the exterior derivative operator d. This operator maps k-forms to (k+1)-forms. For a k-form ω = f dxa over Rn, we define the action of d by:
with extension to general k-forms occurring linearly.
This more general approach allows for a more natural coordinate-free approach to integration on manifolds. It also allows for a natural generalisation of the fundamental theorem of calculus, called Stokes’ theorem, which we may state as
where ω is a general k-form, and ∂Ω denotes the boundary of the region Ω. Thus, in the case that ω is a 0-form and Ω is a closed interval of the real line, this reduces to the fundamental theorem of calculus. In the case that ω is a 1-form and Ω is a two-dimensional region in the plane, the theorem reduces to Green’s theorem. Similarly, using 2-forms, and 3-forms and Hodge duality, we can arrive at Stokes’ theorem and the divergence theorem. In this way we can see that differential forms provide a powerful unifying view of integration.
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Posted on September 5, 2011, in education and tagged Élie Cartan, Differential form, Exterior algebra, Exterior derivative, Fundamental theorem of calculus, Math, Smooth function, Stokes' theorem. Bookmark the permalink. 2 Comments.