## Entropy and Sidorenko’s conjecture — after Szegedy

November 18, 2015

Here is a simple but important fact about bipartite graphs. Let $G$ be a bipartite graph with (finite) vertex sets $X$ and $Y$ and edge density $\alpha$ (meaning that the number of edges is $\alpha |X||Y|$). Now choose $(x_1,x_2)$ uniformly at random from $X^2$ and $(y_1,y_2)$ uniformly at random from $Y^2$. Then the probability that all of $x_1y_1, x_1y_2, x_2y_1$ and $x_2y_2$ are edges is at least $\alpha^4$.

The proof is very simple. For each $x$, let $f_x:Y\to\{0,1\}$ be the characteristic function of the neighbourhood of $x$. That is, $f_x(y)=1$ if $xy$ is an edge and $0$ otherwise. Then $\sum_{x,y}f_x(y)$ is the sum of the degrees of the $x\in X$, which is the number of edges of $G$, which is $\alpha |X||Y|$. If we set $f=\sum_{x\in X}f_x$, then this tells us that $\sum_{y\in Y}f(y)=\alpha|X||Y|$. By the Cauchy-Schwarz inequality, it follows that $\sum_{y\in Y}f(y)^2\geq\alpha^2|X|^2|Y|$.

But by the Cauchy-Schwarz inequality again,
$(\sum_{y\in Y}f(y)^2)^2=(\sum_{y\in Y}\sum_{x_1,x_2\in X}f_{x_1}(y)f_{x_2}(y))^2$
$= (\sum_{x_1,x_2\in X}\sum_{y\in Y}f_{x_1}(y)f_{x_2}(y))^2$
$\leq |X|^2\sum_{x_1,x_2\in X}(\sum_{y\in Y}f_{x_1}(y)f_{x_2}(y))^2$
$=|X|^2\sum_{x_1,x_2\in X}\sum_{y_1,y_2\in Y}f_{x_1}(y_1)f_{x_2}(y_1)f_{x_1}(y_2)f_{x_2}(y_2).$
That last expression is $|X|^2$ times the number of quadruples $x_1,x_2,y_1,y_2$ such that all of $x_1y_1, x_1y_2, x_2y_1$ and $x_2y_2$ are edges, and our previous estimate shows that it is at least $\alpha^4|X|^4|Y|^2$. Therefore, the probability that a random such quadruple consists entirely of edges is at least $\alpha^4$, as claimed (since there are $|X|^2|Y|^2$ possible quadruples to choose from). Read the rest of this entry »

## Interesting times in academic publishing

November 10, 2015

In this post I want briefly to mention four current goings on in the world of academic publishing.

First, I’ll just briefly say that things are going well with the new journal Discrete Analysis, and I think we’re on course to launch, as planned, early next year with a few very good accepted papers — we certainly have a number of papers in the pipeline that look promising to me. Of course, we’d love to have more.

Secondly, a very interesting initiative has recently been started by Martin Eve, called the Open Library of Humanities. The rough idea is that they provide a platform for humanities journals that are free to read online and free for authors (or, as some people like to say, are Diamond OA journals). Perhaps the most interesting aspect of this initiative is that it is funded by a consortium of libraries. Librarians are the people who feel the pain of ridiculous subscription prices, so they have great goodwill towards people who are trying to build new and cheaper publication models. I think there is no reason that the sciences couldn’t do something similar — in fact, it should be even easier to find money.
Read the rest of this entry »

## Gil Kalai starts Polymath10

November 5, 2015

I’ve already mentioned this on Google Plus, but my readership there is different from my readership here, so it seems a good idea to write a similar post here, to give what publicity I can to the fact that Gil Kalai has started a new Polymath project. The problem he would like solved (and I would very much like to see it solved too) is the famous delta-systems or sunflower conjecture of Erdős and Rado.

The problem, as with many problems in combinatorics, is easy to state, but fascinatingly hard to solve. It is a classic extremal problem, in that it asks how big some combinatorial object needs to be before it is guaranteed to contain a subobject of some particular kind. In this case, the object is a $k$uniform hypergraph, which just means a collection of sets of size $k$. The subobject one would like to find is a sunflower of size $r$, which means a collection of sets $A_1,\dots,A_r$ such that we can find disjoint sets $H, P_1,\dots,P_r$ with the $P_i$ disjoint and with $A_i=H\cup P_i$ for each $i$. I have used the letters $H$ and $P$ to stand for “head” and “petal” — $H$ is the head of the sunflower and $P_1,\dots,P_r$ are the petals. Read the rest of this entry »

## EDP28 — problem solved by Terence Tao!

September 20, 2015

I imagine most people reading this will already have heard that Terence Tao has solved the Erdős discrepancy problem. He has blogged about the solution in two posts, a first that shows how to reduce the problem to the Elliott conjecture in number theory, and a second that shows (i) that an averaged form of the conjecture is sufficient and (ii) that he can prove the averaged form. Two preprints covering (i) and (ii) are here and here: the one covering (i) has been submitted to Discrete Analysis.

This post is therefore the final post of the polymath5 project. I refer you to Terry’s posts for the mathematics. I will just make a few comments about what all this says about polymath projects in general.
Read the rest of this entry »

## Discrete Analysis — an arXiv overlay journal

September 10, 2015

This post is to announce the start of a new mathematics journal, to be called Discrete Analysis. While in most respects it will be just like any other journal, it will be unusual in one important way: it will be purely an arXiv overlay journal. That is, rather than publishing, or even electronically hosting, papers, it will consist of a list of links to arXiv preprints. Other than that, the journal will be entirely conventional: authors will submit links to arXiv preprints, and then the editors of the journal will find referees, using their quick opinions and more detailed reports in the usual way in order to decide which papers will be accepted.

Part of the motivation for starting the journal is, of course, to challenge existing models of academic publishing and to contribute in a small way to creating an alternative and much cheaper system. However, I hope that in due course people will get used to this publication model, at which point the fact that Discrete Analysis is an arXiv overlay journal will no longer seem interesting or novel, and the main interest in the journal will be the mathematics it contains.

The members of the editorial board so far — but we may well add further people in the near future — are Ernie Croot, me, Ben Green, Gil Kalai, Nets Katz, Bryna Kra, Izabella Laba, Tom Sanders, Jozsef Solymosi, Terence Tao, Julia Wolf, and Tamar Ziegler. For the time being, I will be the managing editor. I interpret this as meaning that I will have the ultimate responsibility for the smooth running of the journal, and will have to do a bit more work than the other editors, but that decisions about journal policy and about accepting or rejecting papers will be made democratically by the whole editorial board. (For example, we had quite a lot of discussion, including a vote, about the title, and the other editors have approved this blog post after suggesting a couple of minor changes.)

I will write the rest of this post as a series of questions and answers.
Read the rest of this entry »

## Is Nick Clegg a Liberal Democrat?

April 28, 2015

All my life I have found that Liberal Democrat policies (and before that, Liberal-SDP Alliance policies, and before that, Liberal policies) have been, if not a perfect match to my own views, then at least the closest to them. In particular, I am broadly centrist, tilting somewhat to the left, strongly in favour of voting reform, strongly in favour of remaining part of the European Union, and very keen to take much more radical action to combat climate change. However, Nick Clegg is doing his best to persuade me that to vote Liberal Democrat is no longer a good way of promoting these values. Here is what he has said about building coalitions after the election:

As we have always said, the party with the most votes and the most seats in this election has the first right to seek to form a Government. The British people would rightly question the legitimacy of a coalition that didn’t allow the party with the largest number of seats and votes the opportunity to attempt to form a Government first.

I’m proud that the Liberal Democrats have proved we can form a strong and stable coalition government, able to bring prosperity to Britain.

Just like we would not put UKIP in charge of Europe, we are not going to put the SNP in charge of Britain – a country they want to rip apart.

The current projections at Five Thirty-Eight put the Conservatives on 281 seats, Labour on 268, the Scottish Nationalists on 49 and the Liberal Democrats on 26. If these are correct, then Clegg is saying that he will try first to form a Government with the Conservatives. I claim that this is inconsistent with all four of the fundamental Liberal values I mentioned.
Read the rest of this entry »

## USS changes — don’t be fooled

March 12, 2015

This post is meant for anybody who will be affected by proposed changes to the Universities Superannuation Scheme, the body to which I and many other UK academics have paid our pension contributions and that now proposes to change the rules to deal with the fact that it has a large deficit as a result of the financial crisis. (Or rather, it says it has a large deficit, but there are arguments that the amount by which it is in deficit or surplus is highly volatile, so major changes are not necessarily justified.)

Of course, any change will have to be in the direction of making the deal less generous for those with pensions. Indeed, changes have already been made. Until a few years ago, the amount you got at the end was based on your final salary. More precisely, you got one 80th of your final salary per year after retirement for each year that you contributed to the scheme, up to a maximum of 40 years of contributions (and thus a maximum of half your final salary when you retire). But a few years ago they closed this final-salary scheme to new entrants, because (they said) it had become too expensive. This was partly because now a much larger proportion of academics end up as professors, so their final salaries are higher, and also of course because people live for longer.
Read the rest of this entry »

## Sums of kth powers

November 4, 2014

Recently I was giving a talk at a school in London and had cause to think about sums of kth powers, because I wanted to show people the difference between unenlightening proofs and enlightening ones. (My brief was to try to give them some idea of what proofs are and why they are worth bothering about.) On the train down, I realized that there is a very simple general argument that can be used to work out formulae for sums of kth powers. After a brief look online, I haven’t found precisely this method, though I’ve found plenty of methods in the vicinity of it. It’s bound to be in the literature somewhere, so perhaps somebody can point me towards a reference. But even so, it’s worth a short post.

I’ll start with the case $k=1$. I want to phrase a familiar argument in a way that will make it easy to generalize in the way I want to generalize it. Let $R_2(n)$ be the rectangle consisting of all integer points $(x,y)$ such that $1\leq x\leq n$ and $1\leq y\leq n+1$. We can partition $R_2(n)$ into those points for which $x\geq y$ and those points for which $x. The number of points of the first kind is $1 + 2 + ... + n$, since for each $x$ we get $x$ possibilities for $y$. The number of points of the second kind is $0 + 1 + ... + n$, since for each $y$ we get $y-1$ possibilities for $x$. Therefore, $2(1 + 2 + ... + n) = n(n+1)$ and we get the familiar formula. Read the rest of this entry »

## ICM2014 — Bhargava, Gentry, Sanders

September 7, 2014

On my last day at the ICM I ended up going to fewer talks. As on the previous two days the first plenary lecture was not to be missed — it was Maryam Mirzakhani — so despite my mounting tiredness I set my alarm appropriately. I was a little surprised when I got there by just how empty it was, until eventually I saw that on the screens at the front it said that the lecture was cancelled because of her Fields medallist’s lecture the following Tuesday. I belonged to the small minority that had not noticed this, partly because I have had a lot of trouble with my supposedly-smart phone so was there with a temporary and very primitive replacement which was not the kind of phone on to which one could download a special ICM app that kept one up to date with things like this. I had planned to skip the second lecture of the morning, so I slightly rued my lost couple of hours of potential sleep, while also looking forward to being able to use those hours to work, or perhaps make progress with writing these posts — I can’t remember which of the two I ended up doing.

As a result, the first talk I went to was Manjul Bhargava’s plenary lecture, which was another superb example of what a plenary lecture should be like. Like Jim Arthur, he began by telling us an absolutely central general problem in number theory, but interestingly it wasn’t the same problem — though it is related.
Read the rest of this entry »

## ICM2014 — Kollár, Conlon, Katz, Krivelevich, Milnor

September 3, 2014

As the ICM recedes further into the past, these posts start to feel less and less fresh. I’ve had an enforced break from them as over the course of three days I drove my family from the south of France back to Cambridge. So I think I’ll try to do what I originally intended to do with all these posts, and be quite a lot briefer about each talk.

As I’ve already mentioned, Day 3 started with Jim Arthur’s excellent lecture on the Langlands programme. (In a comment on that post, somebody questioned my use of “Jim” rather than “James”. I’m pretty sure that’s how he likes to be known, but I can’t find any evidence of that on the web.) The next talk was by Demetrios Christodoulou, famous for some extraordinarily difficult results he has proved in general relativity. I’m not going to say anything about the talk, other than that I didn’t follow much of it, because he had a series of dense slides that he read word for word. The slides may even have been a suitably chopped up version of his article for the ICM proceedings, but I have not been able to check that. Anyhow, after a gentle introduction of about three or four minutes, I switched off.
Read the rest of this entry »