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.

Let ${\displaystyle \Omega _{c}^{m}(M)}$ denote the space of smooth m-forms with compact support on a smooth manifold ${\displaystyle M.}$ A current is a linear functional on ${\displaystyle \Omega _{c}^{m}(M)}$ which is continuous in the sense of distributions. Thus a linear functional $${\displaystyle T:\Omega _{c}^{m}(M)\to \mathbb {R} }$$ is an m-dimensional current if it is continuous in the following sense: If a sequence ${\displaystyle \omega _{k}}$ of smooth forms, all supported in the same compact set, is such that all derivatives of all their coefficients tend uniformly to 0 when ${\displaystyle k}$ tends to infinity, then ${\displaystyle T(\omega _{k})}$ tends to 0.

The space ${\displaystyle {\mathcal {D}}_{m}(M)}$ of m-dimensional currents on ${\displaystyle M}$ is a real vector space with operations defined by $${\displaystyle (T+S)(\omega ):=T(\omega )+S(\omega ),\qquad (\lambda T)(\omega ):=\lambda T(\omega ).}$$

Much of the theory of distributions carries over to currents with minimal adjustments. For example, one may define the support of a current ${\displaystyle T\in {\mathcal {D}}_{m}(M)}$ as the complement of the biggest open set ${\displaystyle U\subset M}$ such that $${\displaystyle T(\omega )=0}$$ whenever ${\displaystyle \omega \in \Omega _{c}^{m}(U)}$

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

Integration over a compact rectifiable oriented submanifold M (with boundary) of dimension m defines an m-current, denoted by ${\displaystyle [[M]]}$: $${\displaystyle [[M]](\omega )=\int _{M}\omega .}$$

If the boundary ∂M of M is rectifiable, then it too defines a current by integration, and by virtue of Stokes' theorem one has: $${\displaystyle [[\partial M]](\omega )=\int _{\partial M}\omega =\int _{M}d\omega =[[M]](d\omega ).}$$

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 $${\displaystyle \partial :{\mathcal {D}}_{m+1}\to {\mathcal {D}}_{m}}$$ via duality with the exterior derivative by $${\displaystyle (\partial T)(\omega ):=T(d\omega )}$$ for all compactly supported m-forms ${\displaystyle \omega .}$

Certain subclasses of currents which are closed under ${\displaystyle \partial }$ 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.

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

It is possible to define several norms on subspaces of the space of all currents. One such norm is the mass norm. If ${\displaystyle \omega }$ is an m-form, then define its comass by $${\displaystyle \|\omega \|:=\sup\{\left|\langle \omega ,\xi \rangle \right|:\xi {\mbox{ is a unit, simple, }}m{\mbox{-vector}}\}.}$$

So if ${\displaystyle \omega }$ is a simple m-form, then its mass norm is the usual L^∞-norm of its coefficient. The mass of a current ${\displaystyle T}$ is then defined as $${\displaystyle \mathbf {M} (T):=\sup\{T(\omega ):\sup _{x}|\vert \omega (x)|\vert \leq 1\}.}$$

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

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.

Recall that $${\displaystyle \Omega _{c}^{0}(\mathbb {R} ^{n})\equiv C_{c}^{\infty }(\mathbb {R} ^{n})}$$ so that the following defines a 0-current: $${\displaystyle T(f)=f(0).}$$

In particular every signed regular measure ${\displaystyle \mu }$ is a 0-current: $${\displaystyle T(f)=\int f(x)\,d\mu (x).}$$

Let (x, y, z) be the coordinates in ${\displaystyle \mathbb {R} ^{3}.}$ Then the following defines a 2-current (one of many): $${\displaystyle T(a\,dx\wedge dy+b\,dy\wedge dz+c\,dx\wedge dz):=\int _{0}^{1}\int _{0}^{1}b(x,y,0)\,dx\,dy.}$$

• Georges de Rham
• Herbert Federer
• Differential geometry
• Varifold

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