No Arabic abstract
We prove that there are no rigid complex filiform Lie algebras in the variety of (filiform) Lie algebras of dimension less than or equal to 11. More precisely we show that in any Euclidean neighborhood of a filiform Lie bracket (of low dimension), there is a non-isomorphic filiform Lie bracket. This follows by constructing non trivial linear deformations in a Zariski open dense set of the variety of filiform Lie algebras of dimension 9, 10 and 11. (In lower dimensions this is well known.)
For most complex 9-dimensional filiform Lie algebra we find another non isomorphic Lie algebra that degenerates to it. Since this is already known for nilpotent Lie algebras of rank $geq 1$, only the characteristically nilpotent ones should be considered.
We classify, up to isomorphism, gradings by abelian groups on nilpotent filiform Lie algebras of nonzero rank. In case of rank 0, we describe conditions to obtain non trivial $Z_k$-gradings.
In this paper we investigate Leibniz algebras whose quotient Lie algebra is a naturally graded filiform Lie algebra $n_{n,1}.$ We introduce a Fock module for the algebra $n_{n,1}$ and provide classification of Leibniz algebras $L$ whose corresponding Lie algebra $L/I$ is the algebra $n_{n,1}$ with condition that the ideal $I$ is a Fock $n_{n,1}$-module, where $I$ is the ideal generated by squares of elements from $L$.
The notion of rigidity of Lie algebra is linked to the following problem: when does a Lie brackets $mu$ on a vector space g satisfy that every Lie bracket $mu_1$ sufficiently close to $mu$ is of the form $mu_1 = P.mu $ for some P in GL(g) close to the identity? A Lie algebra which satisfies the above condition will be called rigid. The most famous example is the Lie algebra sl(2,C) of square matrices of order $2$ with vanishing trace. This Lie algebra is rigid, that is any close deformation is isomorphic to it. Let us note that, for this Lie algebra, there exists a quantification of its universal algebra. This led to the definition of the famous quantum group SL(2). Another interest of studying the rigid Lie algebras is the fact that there exists, for a given dimension, only a finite number of isomorphic classes of rigid Lie algebras. So we are tempted to establish a classification. This problem has been solved up to the dimension 8. To continue in this direction, properties must be established on the structure of these algebras. One of the first results establishes an algebraicity criterion cite{Carles}. However, the notion of algebraicity which is used is not the classical notion and it includes non-algebraic Lie algebras in the usual sense. The aim of this work is to show that a the Lie algebra is rigid, then its algebra of inner derivations is algebraic.
In this paper we describe central extensions of some nilpotent Leibniz algebras. Namely, central extensions of the Leibniz algebra with maximal index of nilpotency are classified. Moreover, non-split central extensions of naturally graded filiform non-Lie Leibniz algebras are described up to isomorphism. It is shown that $k$-dimensional central extensions ($kgeq 5$) of these algebras are split.