Sharp bounds for the chromatic number of random Kneser graphs
Acta mathematica Universitatis Comenianae, Tome 88 (2019) no. 3, pp. 861-865
Sergei Kiselev; Andrey Kupavskii; Sergei Kiselev; Andrey Kupavskii. Sharp bounds for the chromatic number of random Kneser graphs. Acta mathematica Universitatis Comenianae, Tome 88 (2019) no. 3, pp. 861-865. http://geodesic.mathdoc.fr/item/AMUC_2019_88_3_a77/
@article{AMUC_2019_88_3_a77,
     author = {Sergei Kiselev and Andrey Kupavskii and Sergei Kiselev and Andrey Kupavskii},
     title = { Sharp bounds for the chromatic number of random {Kneser} graphs},
     journal = {Acta mathematica Universitatis Comenianae},
     pages = {861--865},
     year = {2019},
     volume = {88},
     number = {3},
     url = {http://geodesic.mathdoc.fr/item/AMUC_2019_88_3_a77/}
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Given positive integers $n\ge 2k$, a Kneser graph $KG_{n,k}$ is a graph whose vertex set is the collection of all $k$-element subsets of the set $\{1,\ldots, n\}$, with edges connecting pairs of disjoint sets. A famous result due to L. Lov\'asz states that the chromatic number of $KG_{n,k}$ is equal to $n-2k+2$. In this paper, we study the {\it random Kneser graph} $KG_{n,k}(p)$, obtained from $KG_{n,k}$ by including each of the edges of $KG_{n,k}$ independently and with probability $p$. We prove that, for any fixed $k\ge 3$, $\chi(KG_{n,k}(1/2)) = n-\Theta(\sqrt[2k-2]{\log_2 n})$. We also provide new bounds for the case of growing $k$. This significantly improves previous results on the subject, obtained by Kupavskii and by Alishahi and Hajiabolhassan. We also discuss an interesting connection to an extremal problem on embeddability of complexes.