A special case of Hadwiger's conjecture

Abstract

We investigate Hadwiger's conjecture for graphs with no stable set of size 3. Such a graph on at least 2t-1 vertices is not t-1 colorable, so is conjectured to have a $K_t$ minor. There is a strengthening of Hadwiger's conjecture in this case, which states that there is always a minor in which the preimages of the vertices of $K_t$ are connected subgraphs of size one or two. We prove this strengthened version for graphs whose complement has an even number of vertices and fractional chromatic number less than 3. We investigate several possible generalizations and obtain counterexamples for some and improved results from others. We also show that for sufficiently large $n=|V(G)|$, a graph with no stable set of size 3 has a $K_{1/9 n^{4/5}}$ minor using only sets of size one or two as preimages of vertices.