## Ramsey numbers from pseudorandom graphs

One of the foundational results in modern combinatorics is Ramsey’s theorem which states that for every positive integers $s, t$ there exists a constant $n_0$ such that for all $n \geq n_0$, every $2$-coloring of edges of the complete graph $K_n$ has a monochromatic copy of either $K_s$ or $K_t$. The smallest $n_0$ for which the statement holds true is then known as the Ramsey number $R(s, t)$; that is, for all $n \geq R(s, t)$ the statement above is true and there exists a $2$-coloring of the edges of the complete graph on $R(s, t) - 1$ vertices that avoids monochromatic $K_s$ and $K_t$. We know the exact values of $R(s, t)$ only for some small values of $s$ and $t$. A famous example is $R(3,3) = 6$, while despite decades of research we only known that $43 \leq R(5,5) \leq 48$.

Determining the Ramsey numbers for larger values has been widely open since 1940’s. It has become one of the central problems of mathematics to determine the asymptotics of $R(t, t)$ with $t \rightarrow \infty$ and $R(s, t)$ with $s$ fixed and $t \rightarrow \infty$. An easy upper bound follows from a nice inductive prove due to Erdős and Szekeres (1947), and it shows that $R(s, t) \leq \binom{s + t - 2}{s - 1}$. In particular, this implies $R(t,t) = O(4^t/\sqrt{t})$ and $R(s, t) = O(t^{s - 1})$ (with the constant depending on $s$). The best upper bounds that we know are not that far away from these. David Conlon proved in 2016 that there exists a constant $c$ for which $R(t, t) \leq \frac{4^t}{t^{-c \log t/ \log \log t}}$.

Ajtai, Komlós and Szemerédi proved in 1980 that for every fixed $s$ there exists a constant $c_s$ such that $R(s, t) \leq c_s \frac{t^{s - 1}}{(\log t)^{s - 2}}$

For lower bounds, Erdős proved using a probabilistic argument that $R(t, t) \geq (1 - o(1)) \frac{t}{\sqrt{2} e} \sqrt{2}^t$

and the only improvement (since 1947) in that lower bound has been by a factor of $2$.

In case of $R(s, t)$, with $s$ fixed, we know the following lower bound by Bohman and Keevash from 2010 that is again proved probabilistically by carefully analysing a natural random $K_s$-free process: $R(s, t) \geq c'_s \frac{t^{(s + 1)/2}}{(\log t)^{(s + 1)/2 - 1/(s - 2)}}.$

Ignoring the log factors in the bottom, we have $c'_s t^{(s + 1)/2 - o(1)} \leq R(s, t) \leq c_s t^{s - 1 - o(1)}$.

Any improvement in the powers of t here would be ground-breaking. What I want to talk about in this post is a recent work of Dhruv Mubayi and Jacques Verstraete that offers us a new way of possibly improving the lower bound. Briefly, they show that if the main conjecture on pseudorandom graphs that I mentioned in an earlier post is true, then $R(s, t) \geq c''_s t^{s - 1 - o(1)}$ for some constant $c''_s$. Therefore, the existence of the optimal $K_s$-free pseudorandom graphs would give us the correct polynomial in $R(s, t)$.

Here is how their proof goes. Firstly, observe that by taking one of the colour classes to define the edges of a graph, we need to construct an $n$ vertex graph $G$ that has no $K_s$ as subgraphs and has independence number $< t$ to show $R(s, t) \geq n + 1$. The main result that they prove is as follows:

Theorem 1. Let If there exists a $K_s$-free $(n, d, \lambda)$-graph, and $n$ is large enough, then for $t = \lceil 2n \log^2 n/d \rceil$, we have $R(s, t) \geq c_s \frac{n}{\lambda} \log^2 n$.

for some constant $c_s$.

The proof of Theorem 1 is not too hard if you are comfortable with the probabilistic method and you know that you should use Theorem 2.1 from Alon-Rödl. It can be summarised as: “pick each vertex uniformly at random with probability $\log^2 n/(2e^2 \lambda)$ and in the induced subgraph remove one vertex from each independent set of size $t$”.

If the conjecture regarding $K_s$-free pseudorandom graphs is true, then for every integer $s \geq 3$ there exists an infinite sequence of $(n, d, \lambda)$-graphs with $d = \Omega(n^{1-1/(2s - 3)})$ and $\lambda = O(\sqrt{d})$. Consider such a graph $G$ for $n$ large enough. From $t = \lceil 2n \log^2 n/d \rceil$ and $n = O(d^{(2s - 3)/(2s - 4)})$ we get $d = \Omega(t^{2s - 4}/(\log t)^{2(2s - 4)})$. Since $\lambda = O(\sqrt{d})$, from Theorem 1 we have $R(s, t) \geq c_s \frac{n}{\lambda} \log^2n = c_s t \frac{d}{2 \lambda} = \Omega(t \sqrt{d})$, and from the previous expression for $d$ in terms of $t$ we get $R(s, t) = \Omega \left(\frac{t^{s - 1}}{\log^{2s - 4} t} \right).$

So, all we need to do is construct these $K_s$-free pseudorandom graphs. The best construction that we have so far (discussed here) has edge density $\Theta(n^{-1/(s - 1))}$. Sadly, the lower bound that this construction gives us on $R(s, t)$ is $c_s t^{s/2}/(\log t)^{s - 2}$, which is worse than the probabilistic bound of Bohman and Keevash. In general, if we can construction $K_s$-free optimally pseudorandom graphs with density $n^{-1/f(s)}$, we will get the bound $R(s, t) \geq c_s \frac{t^{(f(s) + 1)/2}}{\log^{f(s) - 1} t}$

Therefore, even a construction with density $n^{-1/(s + 1)}$ (or in fact $n^{-1/(s+\epsilon)}$) would already break the Bohman-Keevash lower bound! That should be doable, right? I leave it as an open problem to the readers.

UPDATE 15th Oct 2019: Xiaoyu He has shown that the existence of these optimal $K_s$-free graphs for all $s$ would also determine the multicolour Ramsey numbers $R(s_1, \dots, s_k, t)$ with $s_i$ fixed for all $i$ and $t \rightarrow \infty$, up-to a poly-logarithmic factor. 