WI-posets, graph complexes and Z2-equivalences

An evergreen theme in topological graph theory is the study of graph complexes, (Proof of the Lovász conjecture, arXiv:math.CO/0402395, 2, 2004; J. Combin. Theory Ser. A 25 (1978) 319-324; Using the Borsuk-Ulam Theorem, Lectures on Topological Methods in Combinatorics and Geometry, Springer Universitext, Berlin, 2003; [17]). Many of these complexes are Z2-spaces and the associated Z2-index IndZ2(X) is an invariant of great importance for estimating the chromatic numbers of graphs. We introduce WI-posets (Definition 2) as intermediate objects and emphasize the importance of Bredon's theorem (Theorem 9) which allows us to use standard tools of topological combinatorics for comparison of Z2-homotopy types of Z2-posets. Among the consequences of general results are known and new results about Z2-homotopy types of graph complexes. It turns out that, in spite of great variety of approaches and definitions, all Z2-graph complexes associated to G can be viewed as avatars of the same object, as long as their Z2-homotopy types are concerned. Among the applications are a proof that each finite, free Z2-complex is a graph complex and an evaluation of Z2-homotopy types of complexes Ind(Cn) of independence sets in a cycle Cn.