## Sunday, March 19, 2023

### New Upper Bound on R(k). WOW!

R(k) is the least n such that for all 2-colorings of the edges of $$K_n$$ there is a monochromatic $$K_k$$

(so there are k vertices such that the coloring restricted to the edges between those k vertices is constant)

Here is some history. If I miss a reference or a result, let me know in the comments

$$R(k) \le 2^{2k} = 4^k$$ seems to be folklore and is a very old result. Proof could be shown to HS students. I have. Or 9 year old's who are good at math. I have.

$$R(k)\le {2k-2 \choose k-1}$$ which is approx $$\frac{4^k}{\sqrt k}$$ was shown by Erdos and Szekeres in 1935. Proof could be shown to HS students. I have. Or 9 year old's who are good at math. I have.

Thomson in 1988 got

$$R(k) \le \frac{4^k}{k^A\sqrt{\log k}}$$

for some A. This paper is behind paywalls so will be lost to history and I cannot comment on if the proof is easy or hard. (If you know of a link it, let me know and I will provide it).

Conlon in 2006 got the denom to be super-poly. We omit the result but the paper is here. Proof used the next method of quasi-randomness.

Sah  (his paper is here) optimized Conlon's technique to get

$$R(k) \le 4^{k-c(\log k)^2}.$$

(According to the paper I will point to soon that has the major result, Sah's result is the best one can do with the Thomason-Conlon technique. In Complexity theory we may have formalized that and called it a barrier result.)

These results were interesting and used interesting techniques. However, the best known lower bound is  $$R(k) \ge 2^{k/2}$$ (I've left out constants) so the REAL questions are

a) Can the 4 be replaced with a smaller number in the upper bound?

b) Is there a number a so that R(k) is essentially $$2^{ak}$$?

We note in passing that the lower bound was proven by the Prob Method. That last statement obscures the history--- the Prob Method was invented by Erdos TO PROVE the lower bound. I've seen the prob method called an application of Ramsey Theory which is not quite right. Its more like Ramsey INSPIRED a technique that is used A LOT, including things that are actually practical.

But  back to our story. SO, while all of the above results were interesting, were they making progress towards the REAL question? We can now say YES as progress HAS been made and DID use some of the techniques.

On March 16, 2023 Campos, Griffthis, Morris, Sahasrabudhe posted a paper on arxiv, here, that showed

$$R(k) \le (4-\epsilon)^k$$

for some (quite small) epsilon.

Here are some thoughts which are influenced by my email correspondence with Jacob Fox (an eminent combinatorist) on this topic.

1) I am NOT surprised that the result is true.

2) I AM surprised that it's been proven. Lots of brilliant people have worked on this problem for many years so... .why now? Since its been open for around 70 years, I thought it would take another 70 years to crack.

3) Will this result lead to better results? I hope so!

4) Does R(k) have a nice upper bound? Not clear. The following is unlikely though something like it may be possible:

R(k) roughly$$(3.5)^k$$ for k a power of a Fibonacci prime

R(k) roughly$$(3.8)^k$$othewise

5) (Jacob pointed me to this) There is a paper on Book-Ramsey Numbers that also (on page 2) notes a connection to Ramsey Numbers. The paper is here. A conjecture on Book-Ramsey will lead to a big improvement on Ramsey. Those kinds of results are odd since I can't tell if the upshot is

Lets work hard on Book-Ramsey so we can get better bounds on Ramsey!

or

Book-Ramsey is HARD so lets give up. (Who first said if at first you don't succeed, quit. Why make a damn fool of yourself? ?)

6) The paper with the new result is 57 pages. It looks dense. I will wait for the movie.  I may have to wait a long time.

1. Domotor Palvolgyi12:03 AM, March 20, 2023

If by movie you mean talk that you can watch online, probably not that long.

2. 1) good point.
2) this raises a question which I may blog about later - there are some theorems where seeing a talk on it (even a recorded one so you can't really ask questions) is MUCH BETTER than reading it.

3. Rob Morris is giving recorded lectures showing the details of the proof: https://www.youtube.com/watch?v=GA295heuHoY&list=PLo4jXE-LdDTTUDYMaYoWD0Z3ltOQ_XIoc

1. Great! Now I can see the movie version!

4. I get that the problem's been open a long time and a lot of people have worked on it. But can you give any sense of the broader significance of the result? Does it connect to anything outside of combinatorics?

5. 1) Broader Significance: The techniques used might be. That was what happened with the Prob Method that was first invented to get lower bounds on Ramsey Numbers but is now used everywhere. (2) Does the RESULT connect to anything outside of combinatorics- I would doubt that.