Separation dimension and sparsity

The separation dimension(G) of a hypergraph G is the smallest natural number k for which the vertices of G can be embedded in R-k so that any pair of disjoint edges in G can be separated by a hyperplane normal to one of the axes. Equivalently, it is the cardinality of a smallest family F of total orders of V(G), such that for any two disjoint edges of G, there exists at least one total order in F in which all the vertices in one edge precede those in the other. Separation dimension is a monotone parameter; adding more edges cannot reduce the separation dimension of a hypergraph. In this article, we discuss the influence of separation dimension and edge-density of a graph on one another. On one hand, we show that the maximum separation dimension of a k-degenerate graph on n vertices is O(klglgn) and that there exists a family of 2-degenerate graphs with separation dimension Omega(lglgn). On the other hand, we show that graphs with bounded separation dimension cannot be very dense. Quantitatively, we prove that n-vertex graphs with separation dimension s have at most 3(4lgn)(s-2).n edges. We do not believe that this bound is optimal and give a question and a remark on the optimal bound.