Inversion set of a Weyl-group element (source code)

= Inversion set of a Weyl-group element
{c}
{title2=$N(w)$}
{wiki=Inversion_set}

For a chosen <positive system of a root system> $\Pi$, the inversion set is
$$
N(w)=\{\beta\in\Pi:w(\beta)\in-\Pi\}.
$$
For a finite <Weyl group>, $|N(w)|=\ell(w)$.