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Tim Gowers’ post about Polymath paper published

On January 27, 2009 Tim Gowers’ blog he asked “Is massively collaborative mathematics possible?”. In the blog post he wrote,

“In short, if a large group of mathematicians could connect their brains efficiently, they could perhaps solve problems very efficiently as well.

The next obvious question is this. Why would anyone agree to share their ideas? Surely we work on problems in order to be able to publish solutions and get credit for them. And what if the big collaboration resulted in a very good idea? Isn’t there a danger that somebody would manage to use the idea to solve the problem and rush to (individual) publication?

Here is where the beauty of blogs, wikis, forums etc. comes in: they are completely public, as is their entire history. To see what effect this might have, imagine that a problem was being solved via comments on a blog post. Suppose that the blog was pretty active and that the post was getting several interesting comments. And suppose that you had an idea that you thought might be a good one. Instead of the usual reaction of being afraid to share it in case someone else beat you to the solution, you would be afraid not to share it in case someone beat you to that particular idea. And if the problem eventually got solved, and published under some pseudonym like Polymath, say, with a footnote linking to the blog and explaining how the problem had been solved, then anybody could go to the blog and look at all the comments. And there they would find your idea and would know precisely what you had contributed. There might be arguments about which ideas had proved to be most important to the solution, but at least all the evidence would be there for everybody to look at.”

So, he did just that! He started a polymath project on his blog to tackle a problem that had already been proven but not with an elementary proof, the density Hales-Jewett theorem. Specifically, “a combinatorial approach to density Hales-Jewett, is about one specific idea for coming up with a new proof for the density Hales-Jewett theorem in the case of an alphabet of size 3” which is often referred to in the blog as DHJ(3). In short, combinatorializing the ergodic-theoretic proof of DHJ(3). He wrote,

“Let me briefly try to defend my choice of problem. I wanted to choose a genuine research problem in my own area of mathematics, rather than something with a completely elementary statement or, say, a recreational problem, just to show that I mean this as a serious attempt to do real mathematics and not just an amusing way of looking at things I don’t really care about. This means that in order to have a reasonable chance of making a substantial contribution, you probably have to be a fairly experienced combinatorialist. In particular, familiarity with Szemerédi’s regularity lemma is essential. So I’m not expecting a collaboration between thousands of people, but I can think of far more than three people who are suitably qualified in the above way.”

Things kicked off February 1, 2009 and by March 10, 2009 a solution was being announced! The proof was submitted to the arXiv on October 20, 2009 and now will appear in the Annals of Mathematics.

You can watch it all unfold here:

http://gowers.wordpress.com/category/polymath1/page/2/

Read the proof here:

http://arxiv.org/abs/0910.3926

This is the wiki for polymath projects:

http://michaelnielsen.org/polymath1/index.php?title=Main_Page

This is the polymath blog started by Terence Tao:

http://polymathprojects.org/

In the previous post, two proofs were given for the *Cauchy-Schwarz inequality*. We will now consider another proof.

**Definition 1** Let be an matrix written as a block matrix

where is a matrix, is a matrix, is a , and is a , so . Assuming is nonsingular, then

is called the *Schur complement* of in ; or the *Schur complement* of relative to .

The Schur complement probably goes back to Carl Friedrich Gauss (1777-1855) (for Gaussian elimination). To solve the linear system

that is

by mimicking Gaussian elimination, that is, if is square and nonsingular, then by eliminating , by multiplying the first equation by and subtracting the second equation, we get

Note, the matrix is the Schur complement of in , and if it is square and nonsingular, then we can obtain the solution to our system.

The Schur complement comes up in Issai Schur’s (1875-1941) seminal lemma published in 1917, in which the *Schur determinate formula* was introduced. By considering elementary operations of partitioned matrices, let

where is square and nonsingular. We can change so that the lower-left and upper-right submatrices become . More precisely, we can make the lower-left and upper-right submatrices by subtracting the first row multiplied by from the second row, and by subtracting the first column multiplied by from the second column. In symbols,

and in equation form,

Note that we have obtain the following factorization of :

By taking the determinants

we obtain the Schur’s determinant formula for block matrices,

Mathematician Emilie Virginia Haynsworth (1916-1985) introduced a name and a notation for the Schur complement of a square nonsingular (or invertible) submatrix in a partitioned (two-way block) matrix. The term Schur complement first appeared in Emily’s 1968 paper **On the Schur Complement** in *Basel Mathematical Notes*, then in *Linear Algebra and its Applications Vol. 1* (1968), AMS Proceedings (1969), and in *Linear Algebra and its Applications Vol. 3* (1970).

We will now present a block matrix proof, focusing on complex matrices.

**Proof:** Let . Then

By taking the Schur complement of , we arrive at

and hence

which ensures, when and are square, that

Equality occurs if and only if rank rank ; that is, by the *Guttman rank additivity formula*, rank rank rank if and only if . When is nonsingular, is nonsingular if and only if is nonsingular.

**References**

[1] Zhang, Fuzhen. *Matrix theory: basic results and techniques*. Springer Science & Business Media, 2011.

http://www.phdcomics.com/comics/archive.php?comicid=27

**Definition 1** A vector space over the number field or is called an *inner product space* if it is equipped with an *inner product* satisfying for all and scalar ,

- , if and only if ,
- ,
- , and
- .

is an inner product space over with the inner product

An inner product space over is usually called a *Euclidean space.*

The following properties of an inner product can be deduced from the four axioms in Definition 1:

- ,
- ,
- ,
- for all if and only if , and
- .

An important property shared by all inner products is the *Cauchy-Schwarz inequality* and, for an inner product space, one of the most useful inequalities in mathematics.

**Theorem 1**(Cauchy-Schwarz Inequality) Let be an inner product space. Then for all vectors and in over the field or ,

Equality holds if and only if and are linearly dependent.

The proof of this can be done in a number of different ways. The most common proof is to consider the quadratic function in

and derive the inequality from the non-positive discriminant. We will first present this proof.

**Proof:** Let be given. If , the assertion is trivial, so we may assume that . Let and consider

which is a real quadratic polynomial with real coefficients. Because of axiom (1.), we know that for all real , and hence can have no real simple roots. The discriminant of must therefore be non-positive

and hence

Since this inequality must hold for any pair of vectors, it must hold if is replaced by , so we also have the inequality

But , so

If , then the statement of the theorem is trivial; if not, then we may divide equation (2) by the quantity to obtain the desired inequality

Because of axiom (1.), can have a real (double) root only if for some . Thus, equality can occur in the discriminant condition in equation (1) if and only if and are linearly dependent.

We will now present a matrix proof, focusing on the complex vector space, which is perhaps the simplest proof of the Cauchy-Schwarz inequality.

**Proof:** For any vectors we noticed that,

By taking the determinant for the matrix,

the inequality follows at once,

Equality occurs if and only if the matrix has rank 1; that is, and are linearly dependent.

**References **

[1] Zhang, Fuzhen. *Matrix theory: basic results and techniques*. Springer Science & Business Media, 2011.

Now work to define the terms in the Taniyama-Shimura conjecture is complete, part 4 presents the famous Taniyama-Shimura conjecture which led to a proof of Fermat’s Last Theorem.

We’ve done a lot of work so far just to try to define the terms in the Taniyama-Shimura conjecture, but today we should finally make it. Our last piece of information is to write down what the L-function of a modular form is. Since I don’t want to build a whole bunch of theory needed to define the special class of modular forms we’ll be considering, I’ll just say that we actually need to restrict our definition of “modular form” to “normalized cuspidal Hecke eigenform”. I’ll point out exactly why we need this, but it doesn’t change anything in the conjecture except that every elliptic curve actually corresponds to an even nicer type of modular form.

Let $latex {f\in S_k(\Gamma_0(N))}&fg=000000$ be a weight $latex {k}&fg=000000$ cusp form with $latex {q}&fg=000000$-expansion $latex {\displaystyle f=\sum_{n=1}^\infty a_n q^n}&fg=000000$. Since this is an analytic function on the disk, we have the tools and theorems…

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Taniyama-Shimura 3: L-Series where it will be crucial in the definition of modularity.

For today, we assume our $latex {d}&fg=000000$-dimensional variety $latex {X/\mathbb{Q}}&fg=000000$ has the property that its middle etale cohomology is 2-dimensional. It won’t hurt if you want to just think that $latex {X}&fg=000000$ is an elliptic curve. We will first define the L-series via the Galois representation that we constructed last time. Fix $latex {p}&fg=000000$ a prime not equal to $latex {\ell}&fg=000000$ and of good reduction for $latex {X}&fg=000000$. Let $latex {M=\overline{\mathbb{Q}}^{\ker \rho_X}}&fg=000000$. By definition the representation factors through $latex {{Gal} (M/\mathbb{Q})}&fg=000000$. For $latex {\frak{p}}&fg=000000$ a prime lying over $latex {p}&fg=000000$ the decomposition group $latex {D_{\frak{p}}}&fg=000000$ surjects onto $latex {{Gal} (\overline{\mathbf{F}}_p/\mathbf{F}_p)}&fg=000000$ with kernel $latex {I_{\frak{p}}}&fg=000000$. One of the subtleties we’ll jump over to save time is that $latex {\rho_X}&fg=000000$ acts trivially on $latex {I_{\frak{p}}}&fg=000000$ (it follows from the good reduction assumption), so we can lift the generator of $latex {{Gal} (\overline{\mathbf{F}}_p/\mathbf{F}_p)}&fg=000000$ to get a conjugacy class $latex {{Frob}_p}&fg=000000$ whose image under…

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Taniyama-Shimura 2: Galois Representations where the standard modern approach to defining modularity for other types of varieties.

Fix some proper variety $latex {X/\mathbb{Q}}&fg=000000$. Our goal today will seem very strange, but it is to explain how to get a continuous representation of the absolute Galois group of $latex {\mathbb{Q}}&fg=000000$ from this data. I’m going to assume familiarity with etale cohomology, since describing Taniyama-Shimura is already going to take a bit of work. To avoid excessive notation, all cohomology in this post (including the higher direct image functors) are done on the etale site.

For those that are intimately familiar with etale cohomology, we’ll do the quick way first. I’ll describe a more hands on approach afterwards. Let $latex {\pi: X\rightarrow \mathrm{Spec} \mathbb{Q}}&fg=000000$ be the structure morphism. Fix an algebraic closure $latex {v: \mathrm{Spec} \overline{\mathbb{Q}}\rightarrow \mathrm{Spec}\mathbb{Q}}&fg=000000$ (i.e. a geometric point of the base). We’ll denote the base change of $latex {X}&fg=000000$ with respect to this morphism $latex {\overline{X}}&fg=000000$. Suppose the dimension of $latex {X}&fg=000000$ is $latex {n}&fg=000000$.

Let…

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Great post on understanding the statement of the famous Taniyama-Shimura conjecture that led to the proof of Fermat’s Last Theorem.

It’s time to return to plan A. I started this year by saying I’d post on some fundamental ideas in arithmetic geometry. The local system thing is hard to get motivated about, since the way I was going to use it in my research seems irrelevant at the moment. My other option was to blog some stuff about class field theory, since there is a reading group on the topic that I belong to this quarter.

The first goal of this new series is to understand the statement of the famous Taniyama-Shimura conjecture that led to the proof of Fermat’s Last Theorem. A lot of people can probably mumble something about the conjecture if they have any experience in algebraic/arithmetic geoemtry or any of the number theory type fields, but most people probably can’t say anything precise about what the conjecture says (I’ll continue to call it a “conjecture” even…

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