## 305 – Projects

January 26, 2012

[Edited, April 3, 2012.]

As mentioned on the syllabus,

I expect groups of two or three per project. The deadline for submission is the scheduled time of the final exam. This will constitute 20% or your total grade. Your final project must be typeset; I encourage you to consult with me through the semester in terms of how it looks and its contents.

What I expect is a paper where you explain the topic, and present its history and a few results on it with complete proofs. Work out a few examples. If relevant, do some numerical simulations. List all the references you consult. (Of course, do not plagiarize.) Some of the topics may end up being too ambitious, and if that occurs, let me know. In that case, it would be better to restrict your presentation (to some aspects of the topic at hand) rather than trying to be comprehensive.

I’ll give you a list of references you may find useful once you pick your topic, but of course if you find additional references, use those instead.

Topics:

1. The Banach-Tarski paradox. Chosen by two groups:
• K. Williams.
• J. Giudicelli, Ch. Kelly, and J. Kunz.
2. The 17 plane symmetry groups. Chosen by two groups:
• S. Burns, C. Fletcher, and A. Zell.
• A. Nelson, H. Newman, and M. Shipley.
3. Quaternions and Octonions. Chosen by:
• K. Mcallister.
4. The Gordon game (See 5.5.2 on the book.) Chosen by:
• J. Thompson
5. The $2\times2\times2$ Rubik cube. Chosen by:
• M. Mesenbrink, and N. Stevenson.

## 515 – Homework 1

January 25, 2012

This set is due Feb. 8 at the beginning of lecture. Of course, let me know if more time is needed or anything like that.

## 305 – Homework I

January 23, 2012

This homework set is due Wednesday, February 1st, at the beginning of lecture, but feel free to turn it in earlier if possible.

## 515 – The Dehn-Sydler theorem

January 23, 2012

As mentioned in lecture, Hilbert’s third problem was an attempt to understand whether the Bolyai-Gerwien theorem could generalize to ${\mathbb R}^3:$

## 305 – Solving cubic and quartic equations

January 23, 2012

Ars Magna, “The Great Art”, by Gerolamo Cardano.

## 414/514 – The Schoenberg functions

January 23, 2012

Here is Jeremy Ryder’s project from last term, on the Schoenberg functions. Here we have a space-filling continuous map $f:x\mapsto(\phi_s(x),\psi_s(x))$ whose coordinate functions $\phi_s$ and $\psi_s$ are nowhere differentiable.

The proof that $\phi_s,\psi_s$ are continuous uses the usual strategy, as the functions are given by a series to which Weierstrass $M$-test applies.

The proof that $f$ is space filling is nice and short. The original argument can be downloaded here. A nice graph of the first few stages of the infinite fractal-like process that leads to the graph of $f$ can be seen in page 49 of Thim’s master thesis.

## 414/514 – Faber functions

January 17, 2012

Here is Shehzad Ahmed’s project from last term, on the Faber functions, a family of examples of continuous nowhere differentiable functions. Ahmed’s project centers on one of them, but the argument can be easily adapted to all the functions in the family.

As usual, the function is given as a series $F=\sum_n f_n$ where the functions $f_n$ are continuous, and we can find bounds $M_n$ with $\|f_n\|\le M_n$ and $\sum_n M_n<+\infty$. By the Weierstrass $M$-test, $F$ is continuous. Also, the strategy for nowhere differentiability is typical: We associate with each point $x$ a pair of sequences $(a_n)_{n\ge0}$ and $(b_n)_{n\ge0}$ with $a_n$ strictly decreasing to $x$ and $b_n$ strictly increasing to $x$. The key lemma (shown, for example, in Johan Thim’s Master thesis available here) is that, if a continuous function $f$ is differentiable at $x$, then we have

$\displaystyle f'(x)=\lim_{n\to\infty}\frac{f(a_n)-f(b_n)}{a_n-b_n}.$

In the case of the Faber functions, the functions $f_n$ add `peaks’ in the neighborhood of any point, and the locations of these peaks can be used as the points $a_n$ and $b_n$; moreover, the slopes accumulate at these peaks, and so the limit on the right hand side of the displayed equation above does not converge and, in fact, diverges to $+\infty$ or $-\infty$.

Faber’s original paper, Einfaches Beispiel einer stetigen nirgends differentiierbaren Funktion, Jahresbericht der Deutschen Mathematiker-Vereinigung, (1892) 538-540, is nice to read as well. It is available through the wonderful GDZ, the Göttinger Digitalisierungszentrum.