On the chromatic number of almost s-stable Kneser graphs

Abstract: In 2011, Meunier conjectured that for positive integers $n,k,r,s$ with $ k\geq 2$, $r\geq 2$, and $n\geq \max ({r,s})k$, the chromatic number of $s$ -stable $r$-uniform Kneser hypergraphs is equal to $\left\lceil \frac{n-\max ({r,s})(k-1)}{r-1}\right\rceil $. It is a strengthened version of the conjecture proposed by Ziegler (2002), and Alon, Drewnowski and \L uczak (2009). The problem about the chromatic number of almost $s$-stable $r$ -uniform Kneser hypergraphs has also been introduced by Meunier (2011). For the $r=2$ case of the Meunier conjecture, Jonsson (2012) provided a purely combinatorial proof to confirm the conjecture for $s\geq 4$ and $n$ sufficiently large, and by Chen (2015) for even $s$ and any $n$. The case $ s=3$ is completely open, even the chromatic number of the usual almost $s$ -stable Kneser graphs. In this paper, we obtain a topological lower bound for the chromatic number of almost $s$-stable $r$-uniform Kneser hypergraphs via a different approach. For the case $r=2$, we conclude that the chromatic number of almost $s$-stable Kneser graphs is equal to $n-s(k-1)$ for all $s\geq 2$. Set $t=n-s(k-1)$. We show that any proper coloring of an almost $s$-stable Kneser graph must contain a completely multicolored complete bipartite subgraph $K_{\left\lceil \frac{t}{2}\right\rceil \left\lfloor \frac{t}{2} \right\rfloor }$. It follows that the local chromatic number of almost $s$ -stable Kneser graphs is at least $\left\lceil \frac{t}{2}\right\rceil +1$. It is a strengthened result of Simonyi and Tardos (2007), and Meunier's (2014) lower bound for almost $s$-stable Kneser graphs.