## A quasirandomness implication

November 10, 2018

This is a bit of a niche post, since its target audience is people who are familiar with quasirandom graphs and like proofs of an analytic flavour. Very roughly, a quasirandom graph is one that behaves like a random graph of the same density. It turns out that there are many ways that one can interpret the phrase “behaves like a random graph” and, less obviously, that they are all in a certain sense equivalent. This realization dates back to seminal papers of Thomason, and of Chung, Graham and Wilson.

I was lecturing on the topic recently, and proving that certain of the quasirandomness properties all implied each other. In some cases, the proofs are quite a bit easier if you assume that the graph is regular, and in the past I have sometimes made my life easier by dealing just with that case. But that had the unfortunate consequence that when I lectured on Szemerédi’s regularity lemma, I couldn’t just say “Note that the condition on the regular pairs is just saying that they have quasirandomness property $n$” and have as a consequence all the other quasirandomness properties. So this year I was determined to deal with the general case, and determined to find clean proofs of all the implications. There is one that took me quite a bit of time, but I got there in the end. It is very likely to be out there in the literature somewhere, but I haven’t found it, so it seems suitable for a blog post. I can be sure of at least one interested reader, which is the future me when I find that I’ve forgotten the argument (except that actually I have now found quite a conceptual way of expressing it, so it’s just conceivable that it will stick around in the more accessible part of my long-term memory).
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## Additional thoughts on the Ted Hill paper

September 13, 2018

First, I’d like to thank the large number of commenters on my previous post for keeping the discussion surprisingly calm and respectful given the topic discussed. In that spirit, and to try to practise the scientific integrity that I claimed to care about, I want to acknowledge that my views about the paper have changed somewhat as a result of the discussion. My understanding of the story of what happened to the paper has changed even more now that some of those attacked in Ted Hill’s Quillette article have responded, but about that I only want to repeat what I said in one or two comments on the previous post: that my personal view is that one should not “unaccept” or “unpublish” a paper unless something was improper about the way it was accepted or published, and that that is also the view of the people who were alleged to have tried to suppress Ted Hill’s paper on political grounds. I would also remark that whatever happened at NYJM would not have happened if all decisions had to be taken collectively by the whole editorial board, which is the policy on several journals I have been on the board of. According to Igor Rivin, the policy at NYJM is very different: “No approval for the full board is required, or ever obtained. The approval of the Editor in Chief is not required.” I find this quite extraordinary: it would seem to be a basic safeguard that decisions should be taken by more than one person — ideally many more.

To return to the paper, I now see that the selectivity hypothesis, which I said I found implausible, was actually quite reasonable. If you look carefully at my previous post, you will see that I actually started to realize that even when writing it, and it would have been more sensible to omit that criticism entirely, but by the time it occurred to me that ancient human females could well have been selective in a way that could (in a toy model) be reasonably approximated by Hill’s hypothesis, I had become too wedded to what I had already written — a basic writer’s mistake, made in this case partly because I had only a short window of time in which to write the post. I’m actually quite glad I left the criticism in, since I learnt quite a lot from the numerous comments that defended the hypothesis.
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## Has an uncomfortable truth been suppressed?

September 9, 2018

Update to post, added 11th September. As expected, there is another side to the story discussed below. See this statement about the decision by the Mathematical Intelligencer and this one about the decision taken by the New York Journal of Mathematics.

Further update, added 15th September. The author has also made a statement.

I was disturbed recently by reading about an incident in which a paper was accepted by the Mathematical Intelligencer and then rejected, after which it was accepted and published online by the New York Journal of Mathematics, where it lasted for three days before disappearing and being replaced by another paper of the same length. The reason for this bizarre sequence of events? The paper concerned the “variability hypothesis”, the idea, apparently backed up by a lot of evidence, that there is a strong tendency for traits that can be measured on a numerical scale to show more variability amongst males than amongst females. I do not know anything about the quality of this evidence, other than that there are many papers that claim to observe greater variation amongst males of one trait or another, so that if you want to make a claim along the lines of “you typically see more males both at the top and the bottom of the scale” then you can back it up with a long list of citations.

You can see, or probably already know, where this is going: some people like to claim that the reason that women are underrepresented at the top of many fields is simply that the top (and bottom) people, for biological reasons, tend to be male. There is a whole narrative, much loved by many on the political right, that says that this is an uncomfortable truth that liberals find so difficult to accept that they will do anything to suppress it. There is also a counter-narrative that says that people on the far right keep on trying to push discredited claims about the genetic basis for intelligence, differences amongst various groups, and so on, in order to claim that disadvantaged groups are innately disadvantaged rather than disadvantaged by external circumstances.
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## A new journal in combinatorics

June 4, 2018

This post is to announce that a new journal, Advances in Combinatorics, has just opened for submissions. I shall also say a little about the journal, about other new journals, about my own experiences of finding journals I am happy to submit to, and about whether we are any nearer a change to more sensible systems of dissemination and evaluation of scientific papers.

Advances in Combinatorics is set up as a combinatorics journal for high-quality papers, principally in the less algebraic parts of combinatorics. It will be an arXiv overlay journal, so free to read, and it will not charge authors. Like its cousin Discrete Analysis (which has recently published its 50th paper) it will be run on the Scholastica platform. Its minimal costs are being paid for by the library at Queen’s University in Ontario, which is also providing administrative support. The journal will start with a small editorial board. Apart from me, it will consist of Béla Bollobás, Reinhard Diestel, Dan Kral, Daniela Kühn, James Oxley, Bruce Reed, Gabor Sarkozy, Asaf Shapira and Robin Thomas. Initially, Dan Kral and I will be the managing editors, though I hope to find somebody to replace me in that role once the journal is established. While I am posting this, Dan is simultaneously announcing the journal at the SIAM conference in Discrete Mathematics, where he has just given a plenary lecture. The journal is also being announced by COAR, the Confederation of Open Access Repositories. This project aligned well with what they are trying to do, and it was their director, Kathleen Shearer, who put me in touch with the library at Queen’s.

As with Discrete Analysis, all members of the editorial board will be expected to work: they won’t just be lending their names to give the journal bogus prestige. Each paper will be handled by one of the editors, who, after obtaining external opinions (when the paper warrants them) will make a recommendation to the rest of the board. All decisions will be made collectively. The job of the managing editors will be to make sure that this process runs smoothly, but when it comes to decisions, they will have no more say than any other editor.

The rough level that the journal is aiming at is that of a top specialist journal such as Combinatorica. The reason for setting it up is that there is a gap in the market for an “ethical” combinatorics journal at that level — that is, one that is not published by one of the major commercial publishers, with all the well known problems that result. We are not trying to destroy the commercial combinatorial journals, but merely to give people the option of avoiding them if they would prefer to submit to a journal that is not complicit in a system that uses its monopoly power to ruthlessly squeeze library budgets. Read the rest of this entry »

## Two infinities that are surprisingly equal

September 19, 2017

It has been in the news recently — or rather, the small corner of the news that is of particular interest to mathematicians — that Maryanthe Malliaris and Saharon Shelah recently had an unexpected breakthrough when they stumbled on a proof that two infinities were equal that had been conjectured, and widely believed, to be distinct. Or rather, since both were strictly between the cardinality of the natural numbers and the cardinality of the reals, they were widely believed to be distinct in some models of set theory where the continuum hypothesis fails.

A couple of days ago, John Baez was sufficiently irritated by a Quanta article on this development that he wrote a post on Google Plus in which he did a much better job of explaining what was going on. As a result of reading that, and following and participating in the ensuing discussion, I have got interested in the problem. In particular, as a complete non-expert, I am struck that a problem that looks purely combinatorial (though infinitary) should, according to Quanta, have a solution that involves highly non-trivial arguments in proof theory and model theory. It makes me wonder, again as a complete non-expert so probably very naively, whether there is a simpler purely combinatorial argument that the set theorists missed because they believed too strongly that the two infinities were different.

I certainly haven’t found such an argument, but I thought it might be worth at least setting out the problem, in case it appeals to anyone, and giving a few preliminary thoughts about it. I’m not expecting much from this, but if there’s a small chance that it leads to a fruitful mathematical discussion, then it’s worth doing. As I said above, I am indebted to John Baez and to several commenters on his post for being able to write much of what I write in this post, as can easily be checked if you read that discussion as well.
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## Intransitive dice VII — aiming for further results

August 12, 2017

While Polymath13 has (barring a mistake that we have not noticed) led to an interesting and clearly publishable result, there are some obvious follow-up questions that we would be wrong not to try to answer before finishing the project, especially as some of them seem to be either essentially solved or promisingly close to a solution. The ones I myself have focused on are the following.

1. Is it true that if two random elements $A$ and $B$ of $[n]^n$ are chosen, then $A$ beats $B$ with very high probability if it has a sum that is significantly larger? (Here “significantly larger” should mean larger by $f(n)$ for some function $f(n)=o(n^{3/2})$ — note that the standard deviation of the sum has order $n^{3/2}$, so the idea is that this condition should be satisfied one way or the other with probability $1-o(1)$).
2. Is it true that the stronger conjecture, which is equivalent (given what we now know) to the statement that for almost all pairs $(A,B)$ of random dice, the event that $A$ beats a random die $C$ has almost no correlation with the event that $B$ beats $C$, is false?
3. Can the proof of the result obtained so far be modified to show a similar result for the multisets model?

The status of these three questions, as I see it, is that the first is basically solved — I shall try to justify this claim later in the post, for the second there is a promising approach that will I think lead to a solution — again I shall try to back up this assertion, and while the third feels as though it shouldn’t be impossibly difficult, we have so far made very little progress on it, apart from experimental evidence that suggests that all the results should be similar to those for the balanced sequences model. [Added after finishing the post: I may possibly have made significant progress on the third question as a result of writing this post, but I haven’t checked carefully.]
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## Another journal flips

July 27, 2017

There is widespread (even if not universal) agreement that something is deeply wrong with the current system of academic publishing. The basic point, which has been made innumerable times by innumerable people, is that the really hard parts — the writing of papers, and the peer review and selection of the ones to publish — are done voluntarily by academics, and modern technology makes things like typesetting and dissemination extremely cheap. And yet publishers are making more money than ever before. They do this by insisting that we give them ownership of the content we produce (though many journals will publish papers even if you strike out the part of the contract that hands them this ownership — these days I never agree to give copyright to a publisher, and I urge you not to either), and by bundling their journals together so that libraries are forced into an all-or-nothing decision. (As another aside, I also urge libraries to look closely at what is happening in Germany, where they have gone for the “nothing” option with Elsevier and the world has not come to an end.)

What can be done about this? There are many actions, none of which are likely to be sufficient to bring about major change on their own, but which in combination will help to get us to a tipping point. In no particular order, here are some of them.

1. Create new journals that operate much more cheaply and wait for them to become established.
2. Persuade libraries not to agree to Big Deals with the big publishers.
3. Refuse to publish with, write for, or edit for, the big publishers.
4. Make sure all your work is freely available online.
5. Encourage journals that are supporting the big publishers to leave those publishers and set up in a cheaper and fairer way.

Not all of these are easy things to do, but I’m delighted to report that a small group I belong to, set up by Mark Wilson, has, after approaching a large number of maths journals, found one that was ready to “flip”: the Journal of Algebraic Combinatorics has just announced that it will be leaving Springer. Or if you want to be more pedantic about it, a new journal will be starting, called Algebraic Combinatorics and published by The Mersenne Centre for Open Scientific Publishing, and almost all the editors of the Journal of Algebraic Combinatorics will resign from that journal and become editors of the new one, which will adhere to Fair Open Access Principles.

If you want to see change, then you should from now on regard Algebraic Combinatorics as the true continuation of the Journal of Algebraic Combinatorics, and the Journal of Algebraic Combinatorics as a zombie journal that happens to have a name that coincides with a former real journal. And of course, that means that if you are an algebraic combinatorialist with a paper that would have been suitable for the Journal of Algebraic Combinatorics, you should understand that the reputation of the Journal of Algebraic Combinatorics is being transferred, along with the editorial board, to Algebraic Combinatorics, and you should therefore submit it to Algebraic Combinatorics. This has worked with previous flips: the zombie journal rarely thrives afterwards and in some notable cases has ceased to publish after a couple of years or so.
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## Intransitive dice VI: sketch proof of the main conjecture for the balanced-sequences model

July 25, 2017

I have now completed a draft of a write-up of a proof of the following statement. Recall that a random $n$-sided die (in the balanced-sequences model) is a sequence of length $n$ of integers between 1 and $n$ that add up to $n(n+1)/2$, chosen uniformly from all such sequences. A die $(a_1,\dots,a_n)$ beats a die $(b_1,\dots,b_n)$ if the number of pairs $(i,j)$ such that $a_i>b_j$ exceeds the number of pairs $(i,j)$ such that $a_i. If the two numbers are the same, we say that $A$ ties with $B$.

Theorem. Let $A,B$ and $C$ be random $n$-sided dice. Then the probability that $A$ beats $C$ given that $A$ beats $B$ and $B$ beats $C$ is $\frac 12+o(1)$.

In this post I want to give a fairly detailed sketch of the proof, which will I hope make it clearer what is going on in the write-up.
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## Intransitive dice V: we want a local central limit theorem

May 30, 2017

It has become clear that what we need in order to finish off one of the problems about intransitive dice is a suitable version of the local central limit theorem. Roughly speaking, we need a version that is two-dimensional — that is, concerning a random walk on $\mathbb Z^2$ — and completely explicit — that is, giving precise bounds for error terms so that we can be sure that they get small fast enough.

There is a recent paper that does this in the one-dimensional case, though it used an elementary argument, whereas I would prefer to use Fourier analysis. Here I’d like to begin the process of proving a two-dimensional result that is designed with our particular application in mind. If we are successful in doing that, then it would be natural to try to extract from the proof a more general statement, but that is not a priority just yet.

As people often do, I’ll begin with a heuristic argument, and then I’ll discuss how we might try to sharpen it up to the point where it gives us good bounds for the probabilities of individual points of $\mathbb Z^2$. Much of this post is cut and pasted from comments on the previous post, since it should be more convenient to have it in one place.
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## Intransitive dice IV: first problem more or less solved?

May 27, 2017

I hope, but am not yet sure, that this post is a counterexample to Betteridge’s law of headlines. To back up that hope, let me sketch an argument that has arisen from the discussion so far, which appears to get us close to showing that if $A,B$ and $C$ are three $n$-sided dice chosen independently at random from the sequences model (I will recap some of these definitions in a moment), then the probability that $A$ beats $C$ given that $A$ beats $B$ and $B$ beats $C$ is $1/2+o(1)$. Loosely speaking, the information that $A$ beats $B$ and $B$ beats $C$ tells you nothing about how likely $A$ is to beat $C$. Having given the argument, I will try to isolate a statement that looks plausible, not impossible to prove, and sufficient to finish off the argument.

### Basic definitions

An $n$sided die in the sequence model is a sequence $(a_1,\dots,a_n)$ of elements of $[n]=\{1,2,\dots,n\}$ such that $\sum_ia_i=n(n+1)/2$, or equivalently such that the average value of $a_i$ is $(n+1)/2$, which is of course the average value of a random element of $[n]$. A random $n$-sided die in this model is simply an $n$-sided die chosen uniformly at random from the set of all such dice.

Given $n$-sided dice $A=(a_1,\dots,a_n)$ and $B=(b_1,\dots,b_n)$, we say that $A$ beats $B$ if

$|\{(i,j):a_i>b_j\}|>|\{(i,j):a_i

If the two sets above have equal size, then we say that $A$ ties with $B$.
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