In
mathematics, more particularly in
functional analysis
Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure (e.g. inner product, norm, topology, etc.) and the linear functions defined o ...
,
differential topology, and
geometric measure theory
In mathematics, geometric measure theory (GMT) is the study of geometric properties of sets (typically in Euclidean space) through measure theory. It allows mathematicians to extend tools from differential geometry to a much larger class of surfa ...
, a ''k''-current in the sense of
Georges de Rham
Georges de Rham (; 10 September 1903 – 9 October 1990) was a Swiss mathematician, known for his contributions to differential topology.
Biography
Georges de Rham was born on 10 September 1903 in Roche, a small village in the canton of Vaud in ...
is a
functional on the space of
compactly supported
In mathematics, the support of a real-valued function f is the subset of the function domain containing the elements which are not mapped to zero. If the domain of f is a topological space, then the support of f is instead defined as the smalles ...
differential ''k''-forms, on a
smooth manifold
In mathematics, a differentiable manifold (also differential manifold) is a type of manifold that is locally similar enough to a vector space to allow one to apply calculus. Any manifold can be described by a collection of charts (atlas). One ma ...
''M''. Currents formally behave like
Schwartz distribution
Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functions in mathematical analysis. Distributions make it possible to differentiate functions whose derivatives d ...
s 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 derivative
In mathematics, the directional derivative of a multivariable differentiable (scalar) function along a given vector v at a given point x intuitively represents the instantaneous rate of change of the function, moving through x with a velocity ...
s of delta functions (
multipole
A multipole expansion is a mathematical series representing a function that depends on angles—usually the two angles used in the spherical coordinate system (the polar and azimuthal angles) for three-dimensional Euclidean space, \R^3. Similarly ...
s) spread out along subsets of ''M''.
Definition
Let
denote the space of smooth ''m''-
forms
Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens.
Form also refers to:
*Form (document), a document (printed or electronic) with spaces in which to write or enter data
* ...
with
compact support on a
smooth manifold
In mathematics, a differentiable manifold (also differential manifold) is a type of manifold that is locally similar enough to a vector space to allow one to apply calculus. Any manifold can be described by a collection of charts (atlas). One ma ...
A current is a
linear functional
In mathematics, a linear form (also known as a linear functional, a one-form, or a covector) is a linear map from a vector space to its field of scalars (often, the real numbers or the complex numbers).
If is a vector space over a field , the ...
on
which is continuous in the sense of
distributions. Thus a linear functional
is an ''m''-dimensional current if it is
continuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous ...
in the following sense: If a sequence
of smooth forms, all supported in the same compact set, is such that all derivatives of all their coefficients tend uniformly to 0 when
tends to infinity, then
tends to 0.
The space
of ''m''-dimensional currents on
is a
real
Real may refer to:
Currencies
* Brazilian real (R$)
* Central American Republic real
* Mexican real
* Portuguese real
* Spanish real
* Spanish colonial real
Music Albums
* ''Real'' (L'Arc-en-Ciel album) (2000)
* ''Real'' (Bright album) (2010) ...
vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...
with operations defined by
Much of the theory of distributions carries over to currents with minimal adjustments. For example, one may define the support of a current
as the complement of the biggest
open set
In mathematics, open sets are a generalization of open intervals in the real line.
In a metric space (a set along with a distance defined between any two points), open sets are the sets that, with every point , contain all points that are su ...
such that
whenever
The
linear subspace of
consisting of currents with support (in the sense above) that is a compact subset of
is denoted
Homological theory
Integration over a compact
rectifiable oriented submanifold ''M'' (
with boundary) of dimension ''m'' defines an ''m''-current, denoted by
:
If the
boundary
Boundary or Boundaries may refer to:
* Border, in political geography
Entertainment
* ''Boundaries'' (2016 film), a 2016 Canadian film
* ''Boundaries'' (2018 film), a 2018 American-Canadian road trip film
*Boundary (cricket), the edge of the pla ...
∂''M'' of ''M'' is rectifiable, then it too defines a current by integration, and by virtue of
Stokes' theorem one has:
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
via duality with the exterior derivative by
for all compactly supported ''m''-forms
Certain subclasses of currents which are closed under
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
In mathematics, weak topology is an alternative term for certain initial topologies, often on topological vector spaces or spaces of linear operators, for instance on a Hilbert space. The term is most commonly used for the initial topology of a ...
, which will be further simply called ''weak convergence''. A
sequence
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is calle ...
of currents,
converges to a current
if
It is possible to define several
norms on subspaces of the space of all currents. One such norm is the ''mass norm''. If
is an ''m''-form, then define its comass by
So if
is a
simple
Simple or SIMPLE may refer to:
*Simplicity, the state or quality of being simple
Arts and entertainment
* ''Simple'' (album), by Andy Yorke, 2008, and its title track
* "Simple" (Florida Georgia Line song), 2018
* "Simple", a song by Johnn ...
''m''-form, then its mass norm is the usual L
∞-norm of its coefficient. The mass of a current
is then defined as
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 article describes a theorem concerning the dual of a Hilbert space. For the theorems relating linear functionals to Measure (mathematics), measures, see Riesz–Markov–Kakutani representation theorem.''
The Riesz representation theorem, ...
. This is the starting point of
homological integration.
An intermediate norm is Whitney's ''flat norm'', defined by
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
so that the following defines a 0-current:
In particular every
signed regular measure In mathematics, a regular measure on a topological space is a measure for which every measurable set can be approximated from above by open measurable sets and from below by compact measurable sets.
Definition
Let (''X'', ''T'') be a topolo ...
is a 0-current:
Let (''x'', ''y'', ''z'') be the coordinates in
Then the following defines a 2-current (one of many):
See also
*
Georges de Rham
Georges de Rham (; 10 September 1903 – 9 October 1990) was a Swiss mathematician, known for his contributions to differential topology.
Biography
Georges de Rham was born on 10 September 1903 in Roche, a small village in the canton of Vaud in ...
*
Herbert Federer
Herbert Federer (July 23, 1920 – April 21, 2010) was an American mathematician. He is one of the creators of geometric measure theory, at the meeting point of differential geometry and mathematical analysis.Parks, H. (2012''Remembering Herbert ...
*
Differential geometry
*
Varifold
Notes
References
*
*
*
*
* .
*
{{PlanetMath attribution, id=5980, title=Current
Differential topology
Functional analysis
Generalized functions
Generalized manifolds