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Suppose we have a graph $G$ with $n$ vertices that contains neither a clique of size $3 \log(n)$ nor an independent set of size $3 \log(n)$ (for example $G(n,0.5)$ satisfies this property with with high probability). Is it true that the number of edges of $G$ is at least $n^2/100$, i.e., it cannot be too sparse?

More generally, I would like to know whether such graphs have some kind of pseudo-random properties.

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The answer to your question is yes. It is a result of Erdős and Szemerédi from 1972. We know a little, but non-zero about pseudo-random properties of Ramsey graphs. For example, we know that many different kinds of induced subgraphs appear in such a graph. See, for example, a paper by Alon–Balogh–Kostochka–Samotij and references therein.

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    $\begingroup$ Alternative link: bolyai.hu/~p_erdos/1972-08.pdf Also, are you sure this result is strong enough? Theorem 2 seems to be of the right form but the guarantee is only for some constant multiple of $\log n$, not 3. $\endgroup$ May 16, 2013 at 14:29

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