On the Profile of Multiplicities of Complete Subgraphs

Abstract

Let $G$ be a 2-coloring of a complete graph on $n$ vertices, for sufficiently large $n$. We prove that $G$ contains at least $n^{(\frac{1}{4} - o(1))\log n}$ monochromatic complete subgraphs, thus improving over a lower bound of $n^{0.1576\log n}$ due to Székely [Combinatorica, 4 (1984), pp. 363--372]. We also present lower bounds concerning the number of monochromatic complete subgraphs within certain ranges of sizes, incomparable in nature to lower bounds previously proved by Conlon [Combinatorica, 32 (2012), pp. 171--186]. If furthermore one assumes that the largest monochromatic complete subgraph in $G$ is of size $(\frac{1}{2} + o(1))\log n$ (it is a well known open question whether such graphs exist), then for every constant $0 \le c \le \frac{1}{2}$ we determine (up to low order terms) the number of monochromatic complete subgraphs of size $c \log n$. We do so by proving a lower bound that matches (up to low order terms) a previous upper bound of Székely. For example, the number of monochromatic complete subgraphs of size $\frac{1}{2} \log n$ is $n^{\frac{1}{8}(4 - \log e \pm o(1))\log n} \simeq n^{0.32 \log n}$.