# Higher Lie-Schouten brackets

I thought it would be interesting to point out a geometric construction related to  $$L_{\infty}$$-algebras.  (See earlier post here) Recall that given a Lie algebra $$(\mathfrak{g}, [,] )$$ one can associate on the dual vector space a linear Poisson structure known as the Lie-Poisson bracket.  So, as a  manifold $$(\mathfrak{g}^{*}, \{, \})$$ is a Poisson manifold.  It is convenient to  replace the “classical” language of linear and replace this with a graded condition. That is, if we associate weight one to the coordinates on $$\mathfrak{g}^{*}$$ then the Lie-Poisson bracket is of weight minus one.

The Lie Poisson bracket is very important in deformation quantisation (both formal and C*-algebraic). There are some nice theorems and results that I should point to at some later date.

Now, it is also known that one has an odd version of this known as the Lie-Schouten brackets on $$\Pi \mathfrak{g}^{*}$$. The key difference is the shift in the Grassmann parity of the “linear” coordinates.  Note that this all carries over to Lie super algebras with no problem.  I will drop the prefix super from now on…

So, let us look at the situation for $$L_{\infty}$$-algebras. We understand these either as a series of higher order brackets on a vector space  $$U$$ that satisfies a higher order generalsiation of the Jacobi identities or more conveniently we can understand all this in terms of a homological vector field on the formal manifold $$\Pi U$$.

Definition An $$L_{\infty}$$-algebra is a vector space $$V = \Pi U$$ together with a homological vector field $$Q = (Q^{\delta} + \xi^{\alpha} Q_{\alpha}^{\delta} + \frac{1}{2!} \xi^{\alpha} \xi^{\beta} Q_{\beta \alpha}^{\delta} + \frac{1}{3!} \xi^{\alpha} \xi^{\beta} \xi^{\gamma} Q_{\gamma \beta \alpha}^{\delta} + \cdots) \frac{\partial}{\partial \xi^{\delta}}$$,

where we have picked coordinates on $$\Pi U$$  $$\{ \xi^{\alpha}\}$$. Note that these coordinates are odd as compared to the coordinates on $$U$$. Thus we assign the Grassmann parity $$\widetilde{\xi^{\alpha}} = \widetilde{\alpha} + 1$$  Note that $$Q$$ is odd and that if we restrict to the quadratic part then we are back to Lie algebras.

I will simply state the result, rather than derive it.

Proposition Let $$(\Pi U, Q)$$ be an $$L_{\infty}$$-algebra. Then the formal manifold $$\Pi U^{*}$$ has a homotopy Schouten algebra structure.

Let us pick local coordinates $$\{ \eta_{\alpha}\}$$ on $$\Pi U^{*}$$. Furthermore, we consider this as a graded manifold and attach a weight of one to each coordinate.  A general function,  a  “multivector” has the form

$$X = \stackrel{0}{X} + X^{\alpha} \eta_{\alpha} + \frac{1}{2!}X^{\alpha \beta}\eta_{\beta} \eta_{\alpha} + \cdots$$

The higher Lie-Schouten brackets are given by

$$(X_{1}, X_{2}, \cdots, X_{r}) = \pm Q_{\alpha_{r}\cdots \alpha_{1} }^{\beta}\eta_{\beta}\frac{\partial X_{1}}{\partial \eta_{\alpha_{1}}} \cdots \frac{\partial X_{1}}{\partial \eta_{\alpha_{r}}}$$,

being slack with an overall sign.  Note that with respect to the natural weight the n-bracket has weight (1-n). Thus not unexpectedly, restricting to n=2 gives an odd bracket of weight minus one: up to conventions this is the Lie-Schouten bracket of a Lie algebra.

The above collection of brackets forms an $$L_{\infty}$$-algebra in the “odd super” conventions that satisfies a derivation rule of the product of “multivectors”. Thus the nomenclature homotopy Schouten algebra and higher Lie-Schouten bracket.

A similar statement holds in terms of a  homotopy Poisson algebra on $$U^{*}$$. Here the brackets as skewsymmetric and of  even/odd Grassmann parity for even/odd number of arguments.  (I rather the odd conventions overall).

Now this is quite a new construction and the technical exploration of this nice geometric construction awaits to be explored. How much of the geometric theory associated with Lie algebras and Lie groups carries over to $$L_{\infty}$$-algebras and $$\infty$$-groups is an open question.

Details can be found in Andrew James Bruce ” From $$L_{\infty}$$-algebroids to higher Schouten/Poisson structures”, Reports on Mathematical Physics Vol. 67, (2011), No. 2  (also on the arXiv).

Also see earlier post here on Lie infinity algebroids.