The strong hull property for affine irreducible Coxeter groups of rank 3

Abstract: The conjecture proposed by Gaetz and Gao asserts that the Cayley graph of any Coxeter group possesses the strong hull property. This conjecture has been proved for symmetric groups, hyperoctahedral groups, all right-angled Coxeter groups, and computationally verified for finite Coxeter groups of types $D_4$, $F_4$, $G_2$, and $H_3$. This paper investigates all affine irreducible Coxeter groups of rank 3, specifically those of affine types $\widetilde{A}_2$, $\widetilde{C}_2$, and $\widetilde{G}_2$. By employing key concepts from building theory, we develop novel techniques: first reducing and classifying the convex hull in their Cayley graphs into finitely many cases, then proving the strong hull conjecture for these cases through combinatorial computations. Notably, for the case of affine type $\widetilde{G}_2$, we streamline the proof strategy by reducing it to a corollary of results established for affine type $\widetilde{A}_2$. The reduction techniques developed in this study demonstrate potential for generalization. Their possible algebraic reformulation may not only provide new perspectives for further investigation of this conjecture but also offer methodological insights for algebraic combinatorics and geometric group theory.