A local criterion for Tverberg graphs

The topological Tverberg theorem states that for any prime power q and continuous map from a (d+ 1)(q− 1)-simplex to Rd, there are q disjoint faces Fi of the simplex whose images intersect. It is possible to put conditions on which pairs of vertices of the simplex that are allowed to be in the same face Fi. A graph with the same vertex set as the simplex, and with two vertices adjacent if they should not be in the same Fi, is called a Tverberg graph if the topological Tverberg theorem still work. These graphs have been studied by Hell, Schöneborn and Ziegler, and it is known that disjoint unions of small paths, cycles, and complete graphs are Tverberg graphs. We find many new examples by establishing a local criterion for a graph to be Tverberg. An easily stated corollary of our main theorem is that if the maximal degree of a graph is D, and D(D + 1) < q, then it is a Tverberg graph. We state the affine versions of our results and also describe how they can be used to enumerate Tverberg partitions. ∗The author was a Miller Research Fellow 2009-2011 at UC Berkeley, and gratefully acknowledges support from the Adolph C. and Mary Sprague Miller Institute for Basic Research in Science. 1 ar X iv :1 00 2. 34 47 v2 [ m at h. C O ] 1 3 Se p 20 11