Courant algebroid
In a field of mathematics known as differential geometry, a Courant geometry was originally introduced by Zhang-Ju Liu, Alan Weinstein and Ping Xu in their investigation of doubles of Lie bialgebroids in 1997.[1] Liu, Weinstein and Xu named it after Courant, who had implicitly devised earlier in 1990[2] the standard prototype of Courant algebroid through his discovery of a skew symmetric bracket on , called Courant bracket today, which fails to satisfy the Jacobi identity. Both this standard example and the double of a Lie bialgebra are special instances of Courant algebroids.
Definition
[edit]A Courant algebroid consists of the data a vector bundle with a bracket , a non degenerate fiber-wise inner product , and a bundle map (called anchor) subject to the following axioms:
- Jacobi identity:
- Leibniz rule:
- Obstruction to skew-symmetry:
- Invariance of the inner product under the bracket:
where are sections of and is a smooth function on the base manifold . The map is the composition , with the de Rham differential, the dual map of , and the isomorphism induced by the inner product.
Skew-Symmetric Definition
[edit]An alternative definition can be given to make the bracket skew-symmetric as
This no longer satisfies the Jacobi identity axiom above. It instead fulfills a homotopic Jacobi identity.
where is
The Leibniz rule and the invariance of the scalar product become modified by the relation and the violation of skew-symmetry gets replaced by the axiom
The skew-symmetric bracket together with the derivation and the Jacobiator form a strongly homotopic Lie algebra.
Properties
[edit]The bracket is not skew-symmetric as one can see from the third axiom. Instead it fulfills a certain Jacobi identity (first axiom) and a Leibniz rule (second axiom). From these two axioms one can derive that the anchor map is a morphism of brackets:
The fourth rule is an invariance of the inner product under the bracket. Polarization leads to
Examples
[edit]An example of the Courant algebroid is given by the Dorfman bracket[3] on the direct sum with a twist introduced by Ševera,[4] (1998) defined as:
where are vector fields, are 1-forms and is a closed 3-form twisting the bracket. This bracket is used to describe the integrability of generalized complex structures.
A more general example arises from a Lie algebroid whose induced differential on will be written as again. Then use the same formula as for the Dorfman bracket with an A-3-form closed under .
Another example of a Courant algebroid is a quadratic Lie algebra, i.e. a Lie algebra with an invariant scalar product. Here the base manifold is just a point and thus the anchor map (and ) are trivial.
The example described in the paper by Weinstein et al. comes from a Lie bialgebroid, i.e. a Lie algebroid (with anchor and bracket ), also its dual a Lie algebroid (inducing the differential on ) and (where on the right-hand side you extend the -bracket to using graded Leibniz rule). This notion is symmetric in and (see Roytenberg). Here with anchor and the bracket is the skew-symmetrization of the above in and (equivalently in and ):
Dirac structures
[edit]Given a Courant algebroid with the inner product of split signature (e.g. the standard one ), then a Dirac structure is a maximally isotropic integrable vector subbundle , i.e.
- ,
- ,
- .
Examples
[edit]As discovered by Courant and parallel by Dorfman, the graph of a 2-form is maximally isotropic and moreover integrable if and only if , i.e. the 2-form is closed under the de Rham differential, i.e. is a presymplectic structure.
A second class of examples arises from bivectors whose graph is maximally isotropic and integrable if and only if , i.e. is a Poisson bivector on .
Generalized complex structures
[edit]Given a Courant algebroid with inner product of split signature, a generalized complex structure is a Dirac structure in the complexified Courant algebroid with the additional property
where means complex conjugation with respect to the standard complex structure on the complexification.
As studied in detail by Gualtieri[5] the generalized complex structures permit the study of geometry analogous to complex geometry.
Examples
[edit]Examples are, besides presymplectic and Poisson structures, also the graph of a complex structure .
References
[edit]- ^ Z-J. Liu, A. Weinstein, and P. Xu: Manin triples for Lie Bialgebroids, Journ. of Diff.geom. 45 pp.647–574 (1997).
- ^ T.J. Courant: Dirac Manifolds, Transactions of the American Mathematical Society, vol. 319, pp.631–661 (1990).
- ^ I.Y. Dorfman: Dirac structures of integrable evolution equations, Physics Letters A, vol.125, pp.240–246 (1987).
- ^ P. Ševera: Letters to A. Weinstein Archived 2011-07-19 at the Wayback Machine, unpublished.
- ^ M. Gualtieri: Generalized complex geometry, Ph.D. thesis, Oxford university, (2004)
Further reading
[edit]- Dmitry Roytenberg: Courant algebroids, derived brackets, and even symplectic supermanifolds, PhD thesis Univ. of California Berkeley (1999)