## Archive for October, 2013

### What I did in my summer holidays

October 24, 2013

This post is intended to accomplish several things at once. First and foremost, I want to explain (not just in the post) why I have been interested in Borel determinacy and in the natural proofs barrier. Roughly speaking (or should I say tl;dr?) I think that Martin’s proof of Borel determinacy has features that might just conceivably offer a way past that barrier.

As long-term readers of this blog will be aware, the P versus NP problem is one of my personal mathematical diseases (in Richard Lipton’s sense). I had been in remission for a few years, but last academic year I set a Cambridge Part III essay on barriers in complexity theory, and after marking the essays in June I thought I would just spend an hour or two thinking about the problem again, and that hour or two accidentally turned into about three months (and counting).

The trouble was that I had an idea that has refused to die, despite my best efforts to kill it. Like a particularly awkward virus, it has accomplished this by mutating rapidly, so that what it looks like now is very different from what it looked like at the beginning of the summer. (For example, at that stage I hadn’t thought of trying to model a proof on the proof of Borel determinacy.) So what am I to do?
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### Holding a country to ransom

October 15, 2013

Here is a quick thought about the mathematics of the US shutdown, not to be taken too seriously (the thought I mean — the shutdown obviously is to be taken seriously). It’s for the benefit of anyone who is puzzled that the Tea Party can have such a large influence, and more generally how a political system can be stable when almost nobody likes it. I’m going to prove that in a country of $n$ people, it is possible to devise a democratic system in which $n^\alpha$ of those people control the decisions, where $\alpha=\log 2/\log 3$. For example, in a population of 100,000,000, all you need is a band of fanatics with about 112,000 people — or approximately 0.1% of the population. Although we do not have such a system and the distribution is unlikely, the systems and distributions we do have still allow a minority to have undue influence, and for similar reasons. What I’m about to describe is the extreme case.
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October 7, 2013

### Introduction

The purpose of this post is to add some rigour to what I wrote in the previous post, and in particular to the subsection entitled “Why should we believe that the set of easily computable functions is a ‘random-like’ set?” There I proved that if the Rubik’s-cube-like problem is as hard as it looks, then there can be no polynomial-time-computable property that distinguishes between a random composition of $n^k$ 3-bit scramblers and a purely random Boolean function. This implies that there can be no polynomial-time-computable “simplicity” property that is satisfied by all Boolean functions of circuit complexity at most $n^k$ that is not satisfied by almost all Boolean functions.

I personally find the assumption that the Rubik’s-cube-like problem is hard very plausible. However, if you disagree with me, then I don’t have much more I can say (though see Boaz Barak’s first comment on the previous post). What Razborov and Rudich did was to use a different set of random polynomial-time-computable functions that has a better theoretical backing. They build them out of a pseudorandom function generator, which in turn is built out of a pseudorandom generator, which is known to exist if the discrete logarithm problem is hard. And the discrete logarithm problem is hard if factorizing large integers is hard. Since many people have tried hard to find an algorithm for factorizing large integers, there is some quite strong empirical evidence for this problem’s being hard. It’s true that there are also people who think that it is not hard, but the existence of a pseudorandom generator does not depend on the hardness of factorizing. Perhaps a more significant advantage of the Razborov-Rudich argument is that any pseudorandom generator will do. So the correctness of their conclusion is based on a weaker hypothesis than the one I used earlier.
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### How not to prove that P is not equal to NP

October 3, 2013

This is the first of two posts about the difficulty of proving that P$\ne$NP. The next post will contain yet another discussion (there are many of them online) of the famous paper Natural Proofs, by Razborov and Rudich. This one will set the scene by describing a couple of related but less rigorous arguments. I’m writing them because I’ve been fascinated by the natural proofs result ever since I heard about it on a car journey with Michael Saks about twelve years ago, but up to now I’ve been too lazy to follow its proof in detail. I’m now determined to put that right and writing a couple of blog posts seems a good way of forcing myself to read it properly. Although the proof is short, it has certain aspects that have made it hard for me to get my head round it, so I’ll try to write something considerably longer than what Razborov and Rudich write. I’ll assume knowledge of basic definitions such as Boolean functions, circuits, P, and NP. Also, throughout the posts I’ll write as though $n$ is a fixed large integer, when what I’m really talking about is a sequence of integers that tends to infinity. (For instance, I might say that a function $f:\{0,1\}^n\to\{0,1\}$ can be computed in polynomial time. If $n$ is a fixed integer, then that is a meaningless statement, but one can easily convert it into a meaningful statement about a sequence of Boolean functions on larger and larger domains.)

I have a secondary motivation for the posts, which is to discuss a way in which one might try to get round the natural-proofs barrier. Or rather, it’s to discuss a way in which one might initially think of trying to get round it, since what I shall actually do is explain why a rather similar barrier seems to apply to this proof attempt. It might be interesting to convert this part of the discussion into a rigorous argument similar to that of Razborov and Rudich, which is what prompts me to try to understand their paper properly.

But first let me take a little time to talk about what the result says. It concerns a very natural (hence the name of the paper) way that one might attempt to prove that P does not equal NP. Let $B_n$ be the set of all Boolean functions $f:\{0,1\}^n\to\{0,1\}$. Then the strategy they discuss is to show on the one hand that all functions in $B_n$ that can be computed in fewer than $n^k$ steps have some property of “simplicity”, and on the other hand that some particular function in NP does not have that simplicity property.
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