On the non-existence of sympathetic Lie algebras with dimension less than 25

In this article we investigate the question of the lowest possible dimension that a sympathetic Lie algebra $\mathfrak{g}$ can attain, when its Levi subalgebra ${\mathfrak{g}}_L$ is simple. We establish the structure of the nilradical of a perfect Lie algebra $\mathfrak{g}$, as a ${\mathfrak{g}}_L$-module, and determine the possible Lie algebra structures that one such $\mathfrak{g}$ admits. We prove that, as a ${\mathfrak{g}}_L$-module, the nilradical must decompose into at least 4 simple modules. We explicitly calculate the semisimple derivations of a perfect Lie algebra $\mathfrak{g}$ with Levi subalgebra ${\mathfrak{g}}_L={\mathfrak{s}\mathfrak{l}}_2(\mathbb{C})$ and give necessary conditions for $\mathfrak{g}$ to be a sympathetic Lie algebra in terms of these semisimple derivations. We show that there is no sympathetic Lie algebra of dimension lower than 15, independently of the nilradical’s decomposition. If the nilradical has 4 simple modules, we show that a sympathetic Lie algebra has dimension greater or equal than 25.