Short antichains in root systems, semi-Catalan arrangements, and B-stable subspaces

Dmitri I. Panyushev

Let $\be$ be a Borel subalgebra of a complex simple Lie algebra $\g$. An ideal of $\be$ is called ad-nilpotent, if it is contained in $[\be,\be]$. The generators of an ad-nilpotent…