A polytopal generalization of Sperner's Lemma (with Francis Su & Jesus de Loera), Journal of Combinatorial Theory Series A 100, pp. 1-26, 2002. PDF
Abstract: We prove the following conjecture of Atanassov (Studia Sci. Math. Hungar. 32 (1996), 71–74). Let T be a triangulation of a d-dimensional polytope P with n vertices v1, v2, …, vn. Label the vertices of T by 1, 2, …, n in such a way that a vertex of T belonging to the interior of a face F of P can only be labelled by j if vj is on F. Then there are at least n-d full dimensional simplices of T, each labelled with d+1 different labels. We provide two proofs of this result: a non-constructive proof introducing the notion of a pebble set of a polytope, and a constructive proof using a path-following argument. Our non-constructive proof has interesting relations to minimal simplicial covers of convex polyhedra and their chamber complexes, as in Alekseyevskaya (Discrete Math. 157 (1996), 15–37) and Billera et al. (J. Combin. Theory Ser. B 57 (1993), 258–268).