Root polytopes and abelian ideals

We study the root polytope $\mathcal P_Φ$ of a finite irreducible crystallographic root system $Φ$ using its relation with the abelian ideals of a Borel subalgebra of a simple Lie algebra with root system $Φ$. We determine the hyperplane arrangement corresponding to the faces of codimension 2 of $\mathcal P_Φ$ and analyze its relation with the facets of $\mathcal P_Φ$. For $Φ$ of type $A_n$ or $C_n$, we show that the orbits of some special subsets of abelian ideals under the action of the Weyl group parametrize a triangulation of $\mathcal P_Φ$. We show that this triangulation restricts to a triangulation of the positive root polytope $\mathcal P_Φ^+$.

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