Current (mathematics) Information
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 denote the space of smooth m- forms with compact support on a smooth manifold A current is a linear functional on which is continuous in the sense of distributions. Thus a linear functional
The space of m-dimensional currents on is a real vector space 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 such that
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 ∂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
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, which will be further simply called weak convergence. A sequence 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 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 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
In particular every signed regular measure is a 0-current:
Let (x, y, z) be the coordinates in Then the following defines a 2-current (one of many):
See also
Notes
References
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- Federer, Herbert (1969). Geometric measure theory. Die Grundlehren der mathematischen Wissenschaften. Vol. 153. Berlin–Heidelberg–New York: Springer-Verlag. doi: 10.1007/978-3-642-62010-2. ISBN 978-3-540-60656-7. MR 0257325. Zbl 0176.00801.
- Griffiths, Phillip; Harris, Joseph (1978). Principles of algebraic geometry. Pure and Applied Mathematics. New York: John Wiley & Sons. doi: 10.1002/9781118032527. ISBN 0-471-32792-1. MR 0507725. Zbl 0408.14001.
- Simon, Leon (1983). Lectures on geometric measure theory. Proceedings of the Centre for Mathematical Analysis. Vol. 3. Canberra: Centre for Mathematical Analysis at Australian National University. ISBN 0-86784-429-9. MR 0756417. Zbl 0546.49019.
- Whitney, Hassler (1957). Geometric integration theory. Princeton Mathematical Series. Vol. 21. Princeton, NJ and London: Princeton University Press and Oxford University Press. doi: 10.1515/9781400877577. ISBN 9780691652900. MR 0087148. Zbl 0083.28204..
- Lin, Fanghua; Yang, Xiaoping (2003), Geometric Measure Theory: An Introduction, Advanced Mathematics (Beijing/Boston), vol. 1, Beijing/Boston: Science Press/International Press, pp. x+237, ISBN 978-1-57146-125-4, MR 2030862, Zbl 1074.49011
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