Weak faces of highest weight modules and root systems

Abstract: Chari and Greenstein [Adv. Math. 2009] introduced combinatorial subsets of the roots of a finite-dimensional simple Lie algebra $\mathfrak{g}$ which were important in studying Kirillov-Reshetikhin modules over $U_q(\widehat{\mathfrak{g}})$ and their specializations. Later, Khare [J. Algebra. 2016] studied these subsets for many highest weight $\mathfrak{g}$-modules (in finite type), under the name of weak-$\mathbb{A}$-faces (for a subgroup $\mathbb{A}$ of $(\mathbb{R},+)$), and more generally, $({2};{1,2})$-closed subsets. These notions extend and unify the faces of Weyl polytopes as well as the above combinatorial subsets. In this paper, we consider these 'discrete' notions for all Kac-Moody algebras $\mathfrak{g}$, in four distinguished settings: (a) the weights of an arbitrary highest weight $\mathfrak{g}$-module $V$; (b) the convex hull of the weights of $V$; (c) the weights of the adjoint representation; (d) the roots of $\mathfrak{g}$. For (a) (resp., (b)) for all highest weight $\mathfrak{g}$-modules $V$, we show that the weak-$\mathbb{A}$-faces and $({2};{1,2})$-closed subsets agree, and equal the sets of weights on exposed faces (resp., equal the exposed faces) of the convex hull of weights conv$_{\mathbb{R}}$ wt $V$. This completes the partial progress of Khare in finite type, and is novel in infinite type. Our proofs are type-free and self-contained. For (c), (d) involving the root system, we similarly achieve complete classifications. For all Kac-Moody $\mathfrak{g}$ - interestingly, other than $\mathfrak{sl}_3, \widehat{\mathfrak{sl}_3}$ - we show the weak-$\mathbb{A}$-faces and $({2};{1,2})$-closed subsets agree, and equal Weyl group translates of the sets of weights in certain 'standard faces' (which also holds for highest weight modules). This was proved by Chari and her coauthors for root systems in finite type, but is novel for other types.