%0 Journal Article
%T On the Dynkin index of a principal $\mathfrak{sl}_2$-subalgebra
%A Dmitri I. Panyushev
%J Mathematics
%D 2009
%I arXiv
%X Let $g$ be a simple Lie algebra over an algebraically closed field of characteristic zero. The goal of this note is to prove a closed formula for the Dynkin index of a principal $sl_2$-subalgebra of $g$. The key step in the proof uses the "strange formula" of Freudenthal--de Vries. As an application, we (1) compute the Dynkin index any simple $g$-module regarded as $sl_2$-module and (2) obtain an identity connecting the exponents of $g$ and the dual Coxeter numbers of both $g$ and the Langlands dual $g^\vee$.
%U http://arxiv.org/abs/0903.0398v1