Projection of root systems

Sarah Dijols

Published 2019-04-03Version 1

Let $a$ be a real euclidean vector space of finite dimension and $\Sigma$ a root system in $a$ with a basis $\Delta$. Let $\Theta \subset \Delta$ and $M = M_{\Theta}$ be a standard Levi of a reductive group $G$ such that $a_{\Theta} = a_M\slash a_G$. Let us denote $d$ the dimension of $a_{\Theta}$, i.e the cardinal of $\Delta - \Theta$ and $\Sigma_{\Theta}$ the set of all non-trivial projections of roots in $\Sigma$. We obtain conditions on $\Theta$ such that $\Sigma_{\Theta}$ contains a root system of rank $d$.