Current (mathematics)
# Current (mathematics)

In mathematics, more particularly in functional analysis, differential topology, and geometric measure theory, a k-current in the sense of Georges de Rham is a functional on the space of compactly supported differential k-forms, on a smooth manifold M. Currents formally behave like Schwartz distributions on a space of differential forms, but in a geometric setting, they can represent integration over a submanifold, generalizing the Dirac delta function, or more generally even directional derivatives of delta functions (multipoles) spread out along subsets of M.

## Definition

Let $\Omega _{c}^{m}(M)$ denote the space of smooth m-forms with compact support on a smooth manifold $M.$ A current is a linear functional on $\Omega _{c}^{m}(M)$ which is continuous in the sense of distributions. Thus a linear functional

$T:\Omega _{c}^{m}(M)\to \mathbb {R}$

is an m-dimensional current if it is continuous in the following sense: If a sequence $\omega _{k}$ of smooth forms, all supported in the same compact set, is such that all derivatives of all their coefficients tend uniformly to 0 when $k$ tends to infinity, then $T(\omega _{k})$ tends to 0.

The space ${\mathcal {D}}_{m}(M)$ of m-dimensional currents on $M$ is a real vector space with operations defined by

$(T+S)(\omega ):=T(\omega )+S(\omega ),\qquad (\lambda T)(\omega ):=\lambda T(\omega ).$

Much of the theory of distributions carries over to currents with minimal adjustments. For example, one may define the support of a current $T\in {\mathcal {D}}_{m}(M)$ as the complement of the biggest open set $U\subset M$ such that

$T(\omega )=0$

whenever $\omega \in \Omega _{c}^{m}(U)$

The linear subspace of ${\mathcal {D}}_{m}(M)$ consisting of currents with support (in the sense above) that is a compact subset of $M$ is denoted ${\mathcal {E}}_{m}(M).$

## Homological theory

Integration over a compact rectifiable oriented submanifold M (with boundary) of dimension m defines an m-current, denoted by $[[M]]$:

$[[M]](\omega )=\int _{M}\omega .$

If the boundary ∂M of M is rectifiable, then it too defines a current by integration, and by virtue of Stokes' theorem one has:

$[[\partial M]](\omega )=\int _{\partial M}\omega =\int _{M}d\omega =[[M]](d\omega ).$

This relates the exterior derivative d with the boundary operator ∂ on the homology of M.

In view of this formula we can define a boundary operator on arbitrary currents

$\partial :{\mathcal {D}}_{m+1}\to {\mathcal {D}}_{m}$

via duality with the exterior derivative by

$(\partial T)(\omega ):=T(d\omega )$

for all compactly supported m-forms $\omega .$

Certain subclasses of currents which are closed under $\partial$ can be used instead of all currents to create a homology theory, which can satisfy the Eilenberg–Steenrod axioms in certain cases. A classical example is the subclass of integral currents on Lipschitz neighborhood retracts.

## Topology and norms

The space of currents is naturally endowed with the weak-* topology, which will be further simply called weak convergence. A sequence $T_{k}$ of currents, converges to a current $T$ if

$T_{k}(\omega )\to T(\omega ),\qquad \forall \omega .$

It is possible to define several norms on subspaces of the space of all currents. One such norm is the mass norm. If $\omega$ is an m-form, then define its comass by

$\|\omega \|:=\sup\{\left|\langle \omega ,\xi \rangle \right|:\xi {\mbox{ is a unit, simple, }}m{\mbox{-vector}}\}.$

So if $\omega$ is a simple m-form, then its mass norm is the usual $L^∞-norm$ of its coefficient. The mass of a current $T$ is then defined as

$\mathbf {M} (T):=\sup\{T(\omega ):\sup _{x}|\vert \omega (x)|\vert \leq 1\}.$

The mass of a current represents the weighted area of the generalized surface. A current such that M(T) < ∞ is representable by integration of a regular Borel measure by a version of the Riesz representation theorem. This is the starting point of homological integration.

An intermediate norm is Whitney's flat norm, defined by

$\mathbf {F} (T):=\inf\{\mathbf {M} (T-\partial A)+\mathbf {M} (A):A\in {\mathcal {E}}_{m+1}\}.$

Two currents are close in the mass norm if they coincide away from a small part. On the other hand, they are close in the flat norm if they coincide up to a small deformation.

## Examples

Recall that

$\Omega _{c}^{0}(\mathbb {R} ^{n})\equiv C_{c}^{\infty }(\mathbb {R} ^{n})$

so that the following defines a 0-current:

$T(f)=f(0).$

In particular every signed regular measure $\mu$ is a 0-current:

$T(f)=\int f(x)\,d\mu (x).$

Let (x, y, z) be the coordinates in $\mathbb {R} ^{3}.$ Then the following defines a 2-current (one of many):

$T(a\,dx\wedge dy+b\,dy\wedge dz+c\,dx\wedge dz):=\int _{0}^{1}\int _{0}^{1}b(x,y,0)\,dx\,dy.$

