In a field of mathematics known as differential geometry, a Courant algebroid is a structure which, in a certain sense, blends the concepts of Lie algebroid and of quadratic Lie algebra. This notion, which plays a fundamental role in the study of Hitchin's generalized complex structures, was originally introduced by Zhang-Ju Liu, Alan Weinstein and Ping Xu in their investigation of doubles of Lie bialgebroids in 1997. Liu, Weinstein and Xu named it after Courant, who had implicitly devised earlier in 1990 the standard prototype of Courant algebroid through his discovery of a skew symmetric bracket on
Contents
Definition
A Courant algebroid consists of the data a vector bundle
where φ,ψ,χ are sections of E and f is a smooth function on the base manifold M. D is the combination
Skew-Symmetric Definition
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 T is
The Leibniz rule and the invariance of the scalar product become modified by the relation
The skew-symmetric bracket together with the derivation D and the Jacobiator T form a strongly homotopic Lie algebra.
Properties
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 ρ is a morphism of brackets:
The fourth rule is an invariance of the inner product under the bracket. Polarization leads to
Examples
An example of the Courant algebroid is the Dorfman bracket on the direct sum
where X,Y are vector fields, ξ,η are 1-forms and H 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 A whose induced differential on
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 D) are trivial.
The example described in the paper by Weinstein et al. comes from a Lie bialgebroid, i.e. A a Lie algebroid (with anchor
Dirac structures
Given a Courant algebroid with the inner product
Examples
As discovered by Courant and parallel by Dorfman, the graph of a 2-form ω ∈ Ω2(M) is maximally isotropic and moreover integrable iff dω = 0, i.e. the 2-form is closed under the de Rham differential, i.e. a presymplectic structure.
A second class of examples arises from bivectors
Generalized complex structures
(see also the main article generalized complex geometry)
Given a Courant algebroid with inner product of split signature. A generalized complex structure L → M is a Dirac structure in the complexified Courant algebroid with the additional property
where
As studied in detail by Gualtieri the generalized complex structures permit the study of geometry analogous to complex geometry.
Examples
Examples are beside presymplectic and Poisson structures also the graph of a complex structure J: TM → TM.