## Archive for the ‘polymath5’ Category

### EDP20 — squares and fly traps

September 10, 2010

I think this will be a bit long for a comment, so I’ll make it a post instead. I want to try to say as clearly as I can (which is not 100% clearly) what we know about a certain way of constructing a decomposition of the identity on $\mathbb{Q}.$ Recall from the last post or two that what we want to do is this. Define a square in $\mathbb{N}\times\mathbb{N}$ to be a set of the form $[r,s]^2,$ where by $[r,s]$ I mean the set of all positive integers $n$ such that $r\leq n\leq s.$ Let us identify sets with their characteristic functions. We are trying to find, for any constant $C,$ a collection of squares $S_1,\dots,S_k$ and some coefficients $\lambda_1,\dots,\lambda_k$ with the following properties.

• $C\sum_{i=1}^k|\lambda_i|\leq\sum_{i=1}^k\lambda_it_i,$ where $S_i=[r_i,s_i]^2$ and $t_i=(s_i-r_i+1)$ is the number of points in the interval that defines $S_i,$ or, more relevantly, the number of points in the intersection of $S_i$ with the main diagonal of $\mathbb{N}\times\mathbb{N}.$
• Let $f(x,y)=\sum_i\lambda_iS_i(x,y).$ Then for any pair of coprime positive integers $a,b$ we have $\sum_{n=1}^\infty f(na,nb)=0.$

The second condition tells us that the off-diagonal elements of the matrix you get when you convert the decomposition into a matrix indexed by $\mathbb{Q}_+$ are all zero, and the first condition tells us that we have an efficient decomposition in the sense that we care about. In my previous post I showed why obtaining a collection of squares for a constant $C$ implies that the discrepancy of an arbitrary $\pm 1$ sequence is at least $C^{1/2}.$ In this post I want to discuss some ideas for constructing such a system of squares and coefficients. I’ll look partly at ideas that don’t work, so that we can get a sense of what constraints are operating, and partly at ideas that might have a chance of working. I do not guarantee that the latter class of ideas will withstand even five minutes of serious thought: I have already found many approaches promising, only to dismiss them for almost trivial reasons. [Added later: the attempt to write up even the half promising ideas seems to have killed them off. So in the end this post consists entirely of half-baked ideas that I’m pretty sure don’t work. I hope this will lead either to some new and better ideas or to a convincing argument that the approach I am trying to use to create a decomposition cannot work.]
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### EDP19 — removing some vagueness

September 6, 2010

In the comments on EDP18 we are considering a certain decomposition problem that can be understood in its own right. At various points I have asserted that if we can find a decomposition of a particular kind then we will have a positive solution to EDP. And at various points in the past I have even sketched proofs of this. But I think it would be a good idea to do more than merely sketch a proof. So in this post I shall (I hope) give a completely rigorous derivation of EDP from the existence of an appropriate decomposition. (Well, I may be slightly sketchy in places, but only about details where it is obvious that they can be made precise.) I shall also review some material from earlier posts and comments, rather than giving links.

Representing diagonal matrices

First, let me briefly look again at how the ROD (representation of diagonal) approach works. If $P$ and $Q$ are HAPs, I shall write $P\otimes Q$ for the matrix $A$ such that $A_{xy}=1$ if $(x,y)\in P\times Q$ and 0 otherwise. The main thing we need to know about $P\times Q$ is that $\langle x,(P\otimes Q)x\rangle=(\sum_{i\in P}x_i)(\sum_{j\in Q}x_j)$ for every $x.$

Suppose now that $D$ is a diagonal matrix with diagonal entries $d_1,\dots,d_n$ and that we can write it as $\sum_r\lambda_rP_r\otimes Q_r,$ where each $P_r$ and each $Q_r$ is a HAP. Then

$\sum_r\lambda_r(\sum_{i\in P_r}x_i)(\sum_{j\in Q_r}x_j)=\sum_kd_kx_k^2.$

If $\sum_r|\lambda_r|=c\sum_kd_k$ and $x_k=\pm 1$ for every $k,$ then it follows that there exists $r$ such that

$|\sum_{i\in P_r}x_i||\sum_{j\in Q_r}x_j|\geq c^{-1},$

and from that it follows that there is a HAP $P$ such that $|\sum_{i\in P}x_i|\geq c^{-1/2}.$ So if we can make $c$ arbitrarily small, then EDP is proved.
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### EDP18 — apparently P does not equal NP

September 3, 2010

The title of this post is meant to serve various purposes. First and foremost, it is a cheap trick designed to attract attention. Secondly, and relatedly, it is a nod to the amusing events of the last week or so. [Added later: they were from the last week or so when I wrote that sentence.] But there is a third reason that slightly excuses the first two, which is that the current state of play with the EDP project has a very P$\ne$NP-ish feel to it. Indeed, that has been the case for a while: we are trying to find a clever decomposition of a diagonal matrix, which is a difficult search problem, even though we can be fairly confident that if somebody came up with a good candidate for a decomposition, then checking that it worked would be straightforward. And just in case that is not true, let’s make it trivially true by saying that we are searching for a decomposition that can easily be checked to work. If it can easily be checked to work, then it can easily be checked that it can easily be checked to work (the algorithm being to try to check it and see whether you succeed). But now I want to air a suggestion that reduces the search problem to another one that has similar properties but may be easier.

A brief word also on why I am posting again on EDP despite the fact that we are nowhere near 100 comments on the previous post. The main reason is that, now that the rate of commenting has slowed to a trickle, it is far from clear that the same rules should apply. I think the 100-comment rule was a good sufficient condition for a new post, but now I think I want to add a couple more: if there is something to say and quite a long time has elapsed since the previous post, or if there is something to say that takes a while to explain and is not a direct continuation of the current discussion, then it seems a good idea to have a new post. And both these conditions apply.

[Added later: this is a rather strange post written over a few weeks during which my thoughts on the problem were constantly changing. So everything that I say, particularly early on, should be taken with a pinch of salt as I may contradict it later. One approach to reading the post might be to skim it, read the very end a bit more carefully, and then refer back to the earlier parts if you want to know where various ideas came from.]
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### EDP17 — are we nearly there?

July 18, 2010

Apologies for the attention-seeking title, but that really is the purpose of this post. I want to draw attention to some ideas that are buried in more comments that most people are likely to want to read, because I think there is a chance that all that stands between where we are now and a solution to EDP is a few soluble technical problems. It goes without saying that that chance is distinctly less than 100%, but I think it is high enough for it to be worth my going to some trouble to lay out as precisely as I can what the current approach is and what remains to be done. I’ll try to write it in such a way that it explains what is going on even to somebody who has not read any of the posts or comments so far. The exception to that is that I shall not repeat very basic things such as what the Erdős discrepancy problem is, what a homogeneous arithmetic progression is, etc. For that kind of information, I refer the reader to the front page of the Polymath5 wiki.
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### EDP16 — from AP-discrepancy to HAP-discrepancy?

July 4, 2010

In this post I want to elaborate on a strategy for proving EDP that I discussed in EDP15. Briefly, the idea is to take a representation-of-identity proof of Roth’s AP-discrepancy result and modify it so that it becomes a representation-of-diagonal proof of unbounded HAP-discrepancy.

The first step of this programme was an obvious one: obtain a clean and fully detailed proof in the APs case. That has now been completed, and a write-up can be found here. For the benefit of anyone who is interested in thinking about the next stage but doesn’t feel like reading a piece of formal mathematics, let me give a sketch of the argument here. That way, this post will be self-contained. Once I’ve given the sketch, I’ll say what I can about where we might go from here. It is just possible that we are in sight of the finishing line, but that is unlikely as it would depend on various guesses being correct, various potential technical problems not being actual, various calculations giving strong enough bounds, and so on. Thus, the final part of this post will be somewhat speculative, but I will make it as precise as I can in the hope that it will give rise to a fruitful line of enquiry. (more…)

### EDP15 — finding a diagonal matrix

June 21, 2010

It is not at all clear to me what should now happen with the project to solve the Erdős discrepancy problem. A little while ago, it seems that either the project ran out of steam, or else several people, at roughly the same time, decided that other things they were doing should take higher priority for a while. Perhaps those are two ways of saying the same thing.

But they are not quite the same: as an individual researcher I often give up on problems with the strong intention of returning to them, and I often find that if I do return to them then the break has had an invigorating effect. For instance, it can cause me to forget various ideas that weren’t really working, while remembering the important progress. I imagine these are common experiences. But do they apply to Polymath? Is it possible that the EDP project could regain some of its old vigour and that we could push it forward and make further progress? Is it conceivable that it could go into a different mode, where people contributed to it only occasionally? (A problem with that is that one of the appeals of a polymath project is checking in to see whether there are new ideas, or whether people have reacted to your comments. It is not clear to me that this appeal works if it is substantially slowed down.)

Anyhow, as Terry Tao might put it, this situation can be regarded as an opportunity to add a new datapoint to the general polymath experiment. Recently the following conjunction of circumstances occurred: I found myself on a plane, my laptop battery lasts a fraction of the time it should, and all the films on offer were either unappealing or too appealing (meaning that I’d been looking forward to seeing them but didn’t want to waste them by watching them in aeroplane conditions). I therefore found myself with several hours to think about mathematics. It was just like the old days when I didn’t have a laptop and there would only be a couple of films, both terrible. So I thought about EDP. Now I would like to avail myself of the opportunity to obtain a new datapoint by writing my thoughts, such as they were, down in a post and seeing whether anyone feels like joining or rejoining the discussion. I have plenty of questions, some of which may be fairly easy: I hope that will be a good way of tempting people. (more…)

### EDP13 — quick summary

March 23, 2010

I don’t have any major new themes to talk about in this post. We are continuing to look for ways of decomposing matrices that would give rise to proofs of strengthenings of EDP, but I would say that at the moment we are still at the stage of trying to get a feel for the problem. (If you are reading this and wondering what the decomposition problems are that I am talking about, then I suggest having a look at EDP10 and EDP12.)

The difficulty we face seems to be this. (I do not have full confidence in what I am about to write. It could be that I will change my mind, and it could be that others already have a different and better analysis.) I’ll take for the purposes of illustration the problem of writing a diagonal matrix with unbounded trace as a sum $\sum_i\lambda_iP_i\otimes Q_i$ where the $P_i$ and $Q_i$ are HAPs and $\sum_i|\lambda_i|=1$. After trying a few things, I have started to feel as though it isn’t going to be easy to find a completely explicit construction for this. That is partly because no pattern has jumped out at me from the output from Moses’s experiments, though I have not looked as hard as I might. So the best I can think of to do at the moment is something like this (but almost certainly not precisely this).

1. By some kind of wild guess, choose some HAPs $P_i$ and some positive constants $\lambda_i$.

2. Form the matrix $\sum_i\lambda_iP_i\otimes P_i$, which has trace $\sum_i\lambda_i|P_i|$.

3. Make those choices in such a way that $\sum_i\lambda_i|P_i|$ is significantly larger than $\sum_i\lambda_i$.

4. Prove that it is possible to cancel out the off-diagonal part of this matrix and at most half the diagonal part by subtracting a sum of the form $\sum_j\mu_j Q_j\otimes R_j$, with $\sum_j\mu_j$ also significantly smaller than $\sum_i\lambda_i|P_i|$. (more…)

### EDP12 — representing diagonal maps

March 13, 2010

In this post I want to use my unfair advantage as host of this blog to push an approach that is the one that I am currently most interested in. So let me precede it with the qualification that although I shall present the approach as favourably as I can, it may well be that Moses’s SDP approach or Gil’s polynomial approach will in the end turn out to be more fruitful. I should also add that this new approach makes use of a crucial insight from the SDP approach, which is the idea that one can prove a result about all sequences $(x_n)$, including our troublesome friend 1, -1, 0, 1, -1, 0, … if we bound the discrepancy below by an expression of the form $\sum_ib_ix_i^2$.

Lewis’s theorem

Before I explain the new result, I’d like to discuss a result from the geometry of Banach spaces. (I don’t need to do this, but it is a very nice result and this is a good excuse to write about it. If you just skim this section and take note of the definition of “well spread out” below, that will be enough to follow the rest of the post.) The result is a special case of an important theorem of Dan Lewis, though the proof that I shall sketch is a little different from Lewis’s. (more…)

### EDP11 — the search continues

March 7, 2010

I do not have too much to say about the new programme to use SDP to solve EDP, other than that it is still being actively pursued. One remark I would make is that I have rather forgotten recently about keeping the wiki updated, and SDP is a major omission from it. It would be good to have a theoretical account of the method (which I suppose I could paste quite easily from one of my existing posts), and an experimental page devoted particularly to data connected with this approach, with links to all the matrices, sequences and plots that have been produced.

So far, the data has had some expected features (such as the sequence values $b_m$ tending to be larger when $m$ is smooth) and some puzzling ones (such as the fact that $b_{13}$ is much much larger than $b_{11}$). An initial hope for the method was that the experimental data would give rise to a very precise conjecture, but so far that has not happened. There are, however, various avenues that have not been fully explored, and I still have some hope that we will suddenly stumble on some data that we can fully understand. (more…)

### EDP10 — a new and very promising approach

March 2, 2010

From time to time, there has been an input into this project that has given rise to a burst of optimism (on my part anyway). Perhaps the first was the rapid discovery of very long sequences with low discrepancy, and especially the fact that these sequences had interesting structure to them. (The length led me to hope that the conjecture might be false, or at the very least that it might be possible to construct sequences with extremely slow-growing discrepancy, and the structure led me to hope the opposite.) I’m probably forgetting a few things, but the next one I remember is Terence Tao’s amazing observation that we could restrict attention to multiplicative functions if we were prepared to change the problem very slightly. We then discovered (though we sort of knew it anyway) that multiplicative functions are not easy objects to understand …

Since I posted EDP9, there has been a development that has radically changed my perception of the problem, and I imagine that of anyone else who is following closely what is going on. It began with this comment of Moses Charikar. (more…)