## 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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## Razborov and Rudich’s natural proofs argument

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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## Preprint about theorem-proving program up on arXiv

September 19, 2013

Mohan Ganesalingam and I have just put a preprint up on arXiv entitled “A fully automatic problem solver with human-style output.” In it, we give quite a lot of detail about how the program discussed in this post works. We also talk about our general approach, and why we think it may allow us to solve problems fully automatically that are currently out of reach. (Some fairly easy — for humans — problems fall into this category.) We have also made the program’s output for our current set of test problems available. A quick look at that may be a more efficient way of getting an idea of what the program does than reading the explanation we give in the paper.

If you are interested enough to look at the preprint and find that you spot a typo or more serious error, we would of course be very grateful to be told.

## Determinacy of Borel games V

September 16, 2013

This post is a kind of postscript to the previous four. It consists of miscellaneous observations that shed some light on the proof of Borel determinacy. I’m writing it mainly for my own benefit, but there seems to be no harm in posting it, just in case anyone else finds it interesting.

#### An attempt to lift that sort of works and sort of doesn’t

One way to get some further insight into the proof of Borel determinacy is to look at some auxiliary games that don’t work as lifts. A particularly natural candidate is this. Let $T$ be an infinite (pruned) tree, let $A\subset[T]$ and let $G$ be the game $G(T,A)$. Now define a game $G'$ as follows. Player I plays a move of the form $(x_1,\sigma)$, where $\sigma$ is a strategy for $G$ with first move $x_1$. Player II plays a move of the form $(x_2,\tau)$, where $\tau$ is a strategy for $G$ and $x_2=\tau(x_1)$. Thereafter, the two players must play the strategies $\sigma$ and $\tau$.

Clearly the outcome of this game is decided after the first two moves: if $\sigma$ beats $\tau$ then Player I wins and otherwise Player II wins.
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## Determinacy of Borel games IV

September 10, 2013

To recap briefly, we have defined a notion of lifting from one game to another and embarked on a proof that every Borel game can be lifted to a clopen game. The stages of that proof are to show that every closed game can be lifted to a clopen game, and that the set of games that can be lifted is closed under countable unions. It is straightforward to show that it is also closed under taking complements, so that will prove that all Borel games can be lifted to clopen games. As we saw earlier, if a game can be lifted to a determined game, then it is itself determined. Since clopen games are determined, this will be enough.

We are in the middle of the proof that closed games can be lifted to clopen games. We have defined the game to which we will lift, and shown how to map Player I strategies for the auxiliary game $G'$ to strategies for the original game $G$. So the first thing to do in this post is to show how to map Player II strategies for $G'$ to Player II strategies for $G$.
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## Determinacy of Borel games III

September 5, 2013

My aim in this post (if I have enough space) is to prove that every closed game can be lifted to an open (and therefore, by continuity, which is part of the definition of lifting, clopen) game. Since I am discussing a formal proof, I shall be a little more formal with my definitions than I have been so far. Much of what I write to begin with will be repeating things I have already said in the two previous posts.

#### Trees and paths

Recall the definition of a pruned tree. This is an infinite rooted tree $T$ such that from every vertex there is at least one directed infinite path. (Less formally, if you are walking away from the root, you never get stuck.) Given such a tree $T$, we write $[T]$ for the set of infinite directed paths in $T$. If we are working in $\mathbb{N}^{\mathbb{N}}$, then the tree $T$ we will work with has finite sequences as its vertices, with each sequence $(n_1,\dots,n_k)$ joined to its extensions $(n_1,\dots,n_k,n)$. Then $[T]=\mathbb{N}^{\mathbb{N}}$.
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## Determinacy of Borel games II

August 31, 2013

By the end of the previous post, I had said what a Borel subset of $\mathbb{N}^{\mathbb{N}}$ was, and what a determined subset was. Martin’s theorem is the statement that all Borel sets are determined.

I also commented that an intersection of two determined sets does not have to be determined, which suggests that in order to prove that all Borel sets are determined, we will need to find a clever inductive hypothesis. This hypothesis should be of the form, “All Borel sets of index less than $\alpha$ have property P,” where having property P implies that you are determined, and it is also preserved when you take countable intersections and unions.

Since the property of being determined is quite a strange property, it seems rather unlikely that we will be able to find a much simpler property that does the job. So it is natural to look for a property that is itself related to games and determinacy. But what might such a property be?
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## Determinacy of Borel games I

August 23, 2013

I’m trying to understand the famous result of Donald Martin that Borel games are determined, and by that I mean not just understand the proof line by line, but also understand why the lines are as they are. I have found some nice presentations online, in particular these notes by Shehzad Ahmed and this Masters thesis by Ross Bryant, which have been very helpful, not just in presenting the proofs line by line but also, to some extent, in doing things like explaining certain definitions that would otherwise seem a little strange. (Ahmed’s notes are good for a quick overview and Bryant’s thesis is good if you want full details.) Nevertheless, I think one can go further in that direction and speculate about how Martin came up with his proof. I’m actually talking not about his original 1975 proof but about a simpler argument he discovered about ten years later in a paper that, thanks to the generosity of the AMS, I have not been able to look at. (I am on holiday in France, or I suppose I could have trudged over to my departmental library, but for some reason I find I can never bear to do that even if I’m in Cambridge.)

Understanding a proof in that kind of detailed way takes some effort, and usually everybody has to make that kind of effort for themselves. However, that ought not to be the case in the internet age: in this post I’m going to try to write an account of the proof in a way that makes it possible for others to understand it properly without making much effort. Whether I’ll succeed I don’t know. [Added later: I have now written three posts, and expect to finish in one more. I have got far enough to be confident that I will actually manage to write that fourth post. So at least I will end up with a reasonable understanding of the proof, even if I don't manage to transmit it to anyone else.]

One thing I believe quite strongly is that it is better to be in a position where you can remember the ideas in a way that makes it easy to reconstruct the details than to be in a position where you have all the details in front of you but are less certain of the underlying ideas. So I may skimp on some of the details, but only if I am confident that they really have been reduced to easy exercises.
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