# Current

Let be an -dimensional -manifold with countable basis (cf. also Differentiable manifold) and let , where denotes the vector space of compactly supported differential forms of degree on (cf. also Differential form). Endow with the usual structure of a Fréchet space by declaring that tends to if there exists a compact set such that for all and the coefficients of and all their derivatives tend uniformly to those of .

A current on is an element of the dual space . The idea of currents was introduced by G. de Rham in [a6], to obtain a homology theory including both forms and chains, but a precise definition, see [a7], [a8], became only possible after distributions (cf. also Generalized function) had been introduced by L. Schwartz. See also (the editorial comments to) Differential form, whose notation is used here too.

While exterior products of currents are in general undefined, exterior differentiation can be defined by duality. If the action of a current of degree on a form is denoted by , then one defines the exterior differential by . In particular, the notions of closed and exact currents are defined.

Now, let be a complex manifold. One has the splitting for currents just as for forms.

A theorem of P. Lelong [a4] states that any pure -dimensional analytic subset of a Hermitian complex manifold has locally finite -volume. As a consequence one can define the current of integration over by Here, the integration is over the regular points of (cf. also Analytic set). is a -closed current of bi-dimension . Moreover, is positive, that is, is positive for forms , with and being the volume form on the regular points of . See also [a2], [a5].

Thus, currents can be viewed as an extension of the notion of analytic manifold. This idea has been very fruitful in complex analysis. See e.g. [a1], [a3] and their references.