Journal Home Page Cumulative Index List of all Volumes Complete Contentsof this Volume Journal of Lie Theory 26 (2016), No. 1, 001--010Copyright Heldermann Verlag 2016 On the Variety of Four Dimensional Lie Algebras Laurent Manivel Institut de Mathématiques de Marseille, Université Technopôle Château-Gombert, 39 rue Frédéric Joliot-Curie, 13453 Marseille Cedex 13, France laurent.manivel@math.cnrs.fr [Abstract-pdf] Lie algebras of dimension $n$ are defined by their structure constants, which can be seen as sets of $N=n^2(n-1)/2$ scalars (if we take into account the skew-symmetry condition) to which the Jacobi identity imposes certain quadratic conditions. Up to rescaling, we can consider such a set as a point in the projective space {\bf P}$^{N-1}$. Suppose $n=4$, hence $N=24$. Take a random subspace of dimension $12$ in ${\bf P}^{23}$, over the complex numbers. We prove that this subspace will contain exactly $1033$ points giving the structure constants of some four-dimensional Lie algebras. Among those, $660$ will be isomorphic to ${\bf{gl}}_2$, $195$ will be the sum of two copies of the Lie algebra of one-dimensional affine transformations, $121$ will have an abelian three-dimensional derived algebra, and $57$ will have for derived algebra the three dimensional Heisenberg algebra. This answers a question of Kirillov and Neretin. Keywords: Classification of Lie algebras, irreducible component, degree, resolution of singularities. MSC: 14C17, 14M99, 17B05 [ Fulltext-pdf  (297  KB)] for subscribers only.