Reflections on the recent solution of the cap-set problem I

Sometimes blog posts about recent breakthroughs can be useful because they convey the main ideas of a proof without getting bogged down in the technical details. But the recent solution of the cap-set problem by Jordan Ellenberg, and independently and fractionally later by Dion Gijswijt, both making crucial use of an amazing lemma of Croot, Lev and Pach that was made public a week or so before, does not really invite that kind of post, since the papers are so short, and the ideas so transparent, that it’s hard to know how a blog post can explain them more clearly.

But as I’ve got a history with this problem, including posting about it on this blog in the past, I feel I can’t just not react. So in this post and a subsequent one (or ones) I want to do three things. The first is just to try to describe my own personal reaction to these events. The second is more mathematically interesting. As regular readers of this blog will know, I have a strong interest in the question of where mathematical ideas come from, and a strong conviction that they always result from a fairly systematic process — and that the opposite impression, that some ideas are incredible bolts from the blue that require “genius” or “sudden inspiration” to find, is an illusion that results from the way mathematicians present their proofs after they have discovered them.

From time to time an argument comes along that appears to present a stiff challenge to my view. The solution to the cap-set problem is a very good example: it’s easy to understand the proof, but the argument has a magic quality that leaves one wondering how on earth anybody thought of it. I’m referring particularly to the Croot-Lev-Pach lemma here. I don’t pretend to have a complete account of how the idea might have been discovered (if any of Ernie, Seva or Peter, or indeed anybody else, want to comment about this here, that would be extremely welcome), but I have some remarks.

The third thing I’d like to do reflects another interest of mine, which is avoiding duplication of effort. I’ve spent a little time thinking about whether there is a cheap way of getting a Behrend-type bound for Roth’s theorem out of these ideas (and I’m not the only one). Although I wasn’t expecting the answer to be yes, I think there is some value in publicizing some of the dead ends I’ve come across. Maybe it will save others from exploring them, or maybe, just maybe, it will stimulate somebody to find a way past the barriers that seem to be there.

The L-functions and modular forms database

With each passing decade, mathematics grows substantially. As it grows, mathematicians are forced to become more specialized — in the sense of knowing a smaller fraction of the whole — and the time and effort needed to get to the frontier of what is known, and perhaps to contribute to it, increases. One might think that this process will eventually mean that nobody is prepared to make the effort any more, but fortunately there are forces that work in the opposite direction. With the help of the internet, it is now far easier to find things out, and this makes research a whole lot easier in important ways.

It has long been a conviction of mine that the effort-reducing forces we have seen so far are just the beginning. One way in which the internet might be harnessed more fully is in the creation of amazing new databases, something I once asked a Mathoverflow question about. I recently had cause (while working on a research project with a student of mine, Jason Long) to use Sloane’s database in a serious way. That is, a sequence of numbers came out of some calculations we did, we found it in the OEIS, that gave us a formula, and we could prove that the formula was right. The great thing about the OEIS was that it solved an NP-ish problem for us: once the formula was given to us, it wasn’t that hard to prove that it was correct for our sequence, but finding it in the first place would have been extremely hard without the OEIS.

Should I have bet on Leicester City?

If you’re not British, or you live under a stone somewhere, then you may not have heard about one of the most extraordinary sporting stories ever. Leicester City, a football (in the British sense) team that last year only just escaped relegation from the top division, has just won the league. At the start of the season you could have bet on this happening at odds of 5000-1. Just 12 people availed themselves of this opportunity.

Ten pounds bet then would have net me 50000 pounds now, so a natural question arises: should I be kicking myself (the appropriate reaction given the sport) for not placing such a bet? In one sense the answer is obviously yes, as I’d have made a lot of money if I had. But I’m not in the habit of placing bets, and had no idea that these odds were being offered anyway, so I’m not too cut up about it.

Nevertheless, it’s still interesting to think about the question hypothetically: if I had been the betting type and had known about these odds, should I have gone for them? Or would regretting not doing so be as silly as regretting not choosing and betting on the particular set of numbers that just happened to win the national lottery last week?