## Axiom

Text of comments on the sculpture ‘Axiom’ by Mat Chivers at the unveiling ceremony on 28 October, 2014 at the Andrew Wiles building in Oxford:

https://minhyongkim.files.wordpress.com/2014/12/chivers.pdf

## Probability, Sheet 3, Problem 5

I’m sorry I’ve been falling behind on my blogging. I’ll try to catch up gradually. Here is a discussion of the problem, as I mentioned during the tutorial.

## A vector space of uncountable dimension

Let $V=\{ f: {\bf N}\rightarrow R\}$ be the real vector space of all functions from the natural numbers to the reals. Then $V$ has uncountable dimension. To see this, for each $a>0$, let $f_a$ be the function such that $f_a(n)=a^n$.

Claim: The $f_a$ are linearly independent.

Proof: It suffices to show that any finite collection $\{f_{a_i}\}_{i=1}^n$ with a strictly increasing sequence of $a_i$ are linearly independent.

We prove this by induction on $n$, the fact being clear for $n=1$. Suppose $\sum_{i=1}^nc_if_{a_i}=0$ with $n>1$. The equation means

$\sum_{i=1}^nc_ia_i^m=0$

for all $m$. Thus, $\sum_{i=1}^{n-1}c_i(a_i/a_n)^m+c_n=0$ for all $m$. Now let $m\rightarrow \infty$. This shows that $c_n=0$. Thus, by induction, all $c_i=0$.

We have displayed an uncountable linearly independent collection of functions in $V$. Now, let $\{b_i\}_{i\in I}$ be a basis for $V$. Each $f_a$ can be written as a linear combination of $\{b_i\}_{i\in I(a)}$ for some finite set $I(a)$ of indices, where we choose the set $I(a)$ to be minimal with this property. Since the linear span of any finite set of the $b_i$ is finite-dimensional, for any finite subset $S\subset I$, there are at most finitely many $a$ such that $I(a)=S$. That is, the map $a\mapsto I(a)$ is a finite-to-one map from the positive reals $R_{>0}$ to the finite subsets of $I$. Hence, the set of finite subsets of $I$ must be uncountable. But then $I$ itself must be uncountable. (I leave it as an exercise to show that the set of finite subsets of a countable set is itself countable. You should really write out the proof if you’ve never done it before.)

I might point out that before the tutorials, I was a bit confused myself. That is, the first bit about the $f_a$‘s being an uncountable linearly independent set is rather easy. However, I started hesitating: Still, why can’t there be a countable set of elements in terms of which we can express all of them? After all, the set of coefficients we can use for the expressions is uncountable… So think through again clearly: how is this resolved above?

As a final remark, note that this proves that $V$ is not isomorphic to $R[x]$. This is perhaps the first example you’ve seen where you can prove that two vector spaces of *infinite* dimensions are not isomorphic by simply counting the dimensions and comparing them.

## Linear independence of polynomials

One of the exercises this week asked for a proof of linear independence for the set

$\{x^i\}_{i\in {\bf N}}$

inside the polynomials $R[x]$ with real coefficients. However, note that the polynomials here are regarded as *functions* from $R$ to $R$. Thus, it amounts to showing that if

$c_0+c_1x+\cdots c_nx^n=0$

as a function, then all $c_i$ have to be zero. This does require proof. One quick way to do this is to note that all polynomial functions are differentiable. And if

$f(x)=c_0+c_1x+\cdots c_nx^n$

is the zero function, then so are all its derivatives. In particular,

$f^{(i)}(0)=0$

for all $i$. But $f^{(i)}(0)=i!c_i.$ Thus, $c_i=0$ for all $i$.

One possible reason for confusion is that there is another ‘formal’ definition of $R[x]$ by simply identifying a polynomial with its sequence of coefficients. That is, you can think of an element of $R[x]$ as a function $f:N \rightarrow R$ that has *finite support* in that $f(i)=0$ for all but finitely many $i$. With this definition, the polynomial $x^i$ becomes identified with the function $e_i$ that sends $i$ to 1 and everything else to zero. If you take this approach, the linear independence also becomes formal. But in this problem, you are defining $R[x]$ as a function in its variable. This of course is the natural definition you’ve been familiar with at least since secondary school.

Here are two questions:

1. If you think of two polynomials $f$ and $g$ as functions from $N$ to $R$ with finite support, what is a nice way to write the product $fg$?

2. What is the advantage of this formal definition?

## Mathematics in Society

After several conversations recently about the social status of mathematics, I thought I’d put here links to two short essays I wrote on this.

1. An exchange on Mathoverflow

## Mathematical problems

This week, I was given a very short deadline to write one of those introductory blurbs to the Korean edition of Ian Stewart’s book, ‘The Great Mathematical Problems.’ I could only skim through it, but I managed to get enough of a sense to write something. Here it is, in case some students find it interesting.

## SNU Topology of number fields, week of 15 July

I was looking over the notes and saw a few items omitted by Milne.

First, in the proof of Hilbert’s theorem 90, there is no proof of the linear independence of distinct characters on a group. You can try to figure it out yourself (quite easy) or look at the notes with that title Keith Conrad’s webpage. (For some strange reason, I can’t create a direct link to the paper.)

Secondly, Milne has a cryptic remark without definition or proof about a ‘Verlagerung’ (transfer map) in Proposition 3.2. This is reference to the fact that the restriction map for Tate cohomology in degree minus 2 corresponds to a classical formula for a map between abelianizations. This proof is not so straightforward, and can be found in Prop.11.12 of these nice notes by Holden Lee. It seems they’re from a reading course he took with Professor Shin Sug Woo.

If some of you would like to read a more leisurely exposition of group cohomology, I found online these notes by Kenneth Brown.

Finally, this old paper of Oort contains a discussion of extensions and cup products.

## SNU Topology of Number Fields

1. General Motivation:

Lecture at the Cambridge workshop on non-abelian fundamental groups in arithmetic geometry (2009)

Lecture at Bordeaux conference in honor of Martin Taylor’s 60th birthday (2012)

Lecture at British Mathematical Colloquium (2011)

Lecture at AMC, Busan (2013)

2. James Milne’s lecture notes:

Class Field Theory

Algebraic Number Theory

Arithmetic Duality Theorems

3. Langlands programme:

Langlands’s lecture at Helsinki ICM

Report on work of Bao Chau Ngo (with Sugwoo Shin)

4. Topology of number fields:

Mazur’s paper on etale cohomology of number fields

Morishita’s paper on knots and primes

Furusho’s paper on Galois action on knots