Dirac structure

In mathematics a Dirac structure is a geometric construction generalizing both symplectic structures and Poisson structures, and having several applications to mechanics. It is based on the notion of constraint introduced by Paul Dirac and was first introduced by Ted Courant and Alan Weinstein.

In more detail, let V be a real vector space, and V* its dual. A (linear) Dirac structure on V is a linear subspace D of satisfying

  • for all one has ,
  • D is maximal with respect to this property.

In particular, if V is finite dimensional then the second criterion is satisfied if . (Similar definitions can be made for vector spaces over other fields.)

An alternative (equivalent) definition often used is that satisfies , where orthogonality is with respect to the symmetric bilinear form on given by

Examples

  1. If is a vector subspace, then is a Dirac structure on , where is the annihilator of ; that is, .
  2. Let be a skew-symmetric linear map, then the graph of is a Dirac structure.
  3. Similarly, if is a skew-symmetric linear map, then its graph is a Dirac structure.


A Dirac structure on a manifold M is an assignment of a (linear) Dirac structure on the tangent space to M at m, for each . That is,

  • for each , a Dirac subspace of the space .

Many authors, in particular in geometry rather than the mechanics applications, require a Dirac structure to satisfy an extra integrability condition as follows:

  • suppose are sections of the Dirac bundle () then

In the mechanics literature this would be called a closed or integrable Dirac structure.

Examples

  1. Let be a smooth distribution of constant rank on a manifold M, and for each let , then the union of these subspaces over m forms a Dirac structure on M.
  2. Let be a symplectic form on a manifold , then its graph is a (closed) Dirac structure. More generally this is true for any closed 2-form. If the 2-form is not closed then the resulting Dirac structure is not closed (integrable).
  3. Let be a Poisson structure on a manifold , then its graph is a (closed) Dirac structure.

Applications

Port-Hamiltonian systems


Nonholonomic constraints


References

  • H. Bursztyn, A brief introduction to Dirac manifolds. Geometric and topological methods for quantum field theory, 4–38, Cambridge Univ. Press, Cambridge, 2013.
  • Bursztyn, Henrique; Crainic, Marius (2005). "Dirac structures, momentum maps, and quasi-Poisson manifolds". The Breadth of Symplectic and Poisson Geometry. Progress in Mathematics. Vol. 232. Birkhauser-Verlag. pp. 1–40.
  • Courant, Theodore; Weinstein, Alan (1988). "Beyond Poisson structures". Séminaire sud-rhodanien de géométrie VIII. Travaux en Cours. Vol. 27. Paris: Hermann.
  • Dorfman, Irène (1993). Dirac structures and integrability of nonlinear evolution equations. Wiley.
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