Still alive…

but not teaching math at Yale, for all of you JoCo fans. Goal for the summer, in addition to passing comps, is blogging about math. I have some ideas, but we’ll see how they pan out.

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So, umm, yea…

I still exist, despite not posting for almost a year and a half. Go. Me.

While I still intend to (some day) continue with quandle posts, for the time being I’ll be switching to talking about the work I’m doing this summer (and probably will talk a bit about most of the work I did last summer, which is very tangentially related.)

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Kludging an MP3 Bookmark

Suppose you, a run of the mill Linux user, just downloaded a very long mp3 – for example a 10 hour mp3 of Cory Doctorow reading The Hacker Crackdown. Naturally you’re not going listen to this in one sitting; but, you cannot find a media player that implements bookmarking. What do you do?

Continue reading

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Well-Definededness

In Algebra, we just finished chapter 10 of Gallian. One of the big theorems in the chapter is the First Isomorphism Theorem – and since I’m a lazy typist I’ll call it FIT from now on. (Isomorphism theorems 2 and 3 are left as exercises.) Skipping over what a group is, what a homomorphism is, what the kernel of said homomorphism is, and what normal subgroups and factors groups are (see, FIT talks about all sorts of cool stuff!), FIT says:

Suppose \theta :G \longrightarrow G^\prime is a homomorphism. Then G/\text{ker}\theta \cong G^\prime\theta; moreover, the map \theta : G/\text{ker}\theta \longrightarrow G^\prime\theta by g\text{ker}\theta \mapsto g\theta is an isomorphism.

One of the points of the proof, since \theta is defined on the cosets of G/\text{ker}\theta by specifying the image of a representative, is whether or not the map \theta is even well defined for some g\text{ker}\theta \in G/\text{ker}\theta. In particular, will taking different representatives of the coset g\text{ker}\theta always have the same image? It turns out that the map is indeed well defined – it’s a rather straightforward argument – but this just brings up another question:

If a map is well defined, does it have the property of well-definededness?

Yay, English!

EDIT: Fixed my accidental interspersing of \varphi for \theta.

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Quandles

Since I have to begin somewhere, I might as well start with something I find interesting. Back in the summer of 2005, I participated in an REU; our research group (one of four or so) studied objects called quandles. I’ll go into the specifics of what we were trying to do in a later post — today I just want explain what a quandle is.

This being my first mathy post, it’s probably obvious that quandles are a pet topic of mine. And no, I don’t like them just because they may have the best name in a technical field this side of quarks (future nomenclaturists take note – both begin with q.) Joking aside quandles fascinate me because, as we will learn in a moment, quandles are nonassociative. We’ll see, in later posts, that this makes quandles behave quite differently than objects like groups and vector spaces that we may be familiar with.

I’ll talk in more depth about the motivations for studying quandles at a later point; suffice it to say that quandles arise in a very natural way in the context of knot theory. For those of you familiar with knot theory, with the proper setup quandles encode the Reidermeister moves. Quandles also have a significant connection with the fundamental group of the knot complement, if I ever get to algebraic geometry I’ll talk about this connection.[1]

So what exactly are quandles? They are an algebraic system like groups. If you don’t know what a group is, I’ll get there eventually. In defining a quandle, we start with a set Q and a closed binary operation \vartriangleright : Q \times Q \rightarrow Q. We then impose the following axioms:

    \forall a,b,c \in Q the following holds:

  1. a \vartriangleright a = a;
  2. \exists x \in Q such that a \vartriangleright x = b; and lastly,
  3. (a \vartriangleright b) \vartriangleright c = (a \vartriangleright c) \vartriangleright (b \vartriangleright c).

For those of you familiar with more mainstream things like group theory, these axioms probably seem strange. We’ll start with axiom 1. This is like saying that \forall x \in \mathbb{Z}: x + x = x! This is certainly true for 0: 0+0 = 0. But for every other integer, it’s absured. The reason is that in things like groups we can prove:

Theorem:

Let (G, \cdot) be a group and let x \in G. Then x \cdot x = x \implies x = \text{id}_G.

I’m going to withhold the proof of this until the groups section, but if you’re familiar with groups, this is a very straightforward thing to prove.

Put group theoretically: in groups only the the identity element has order 1; in quandles every element has order 1. This axiom will give us some quaint strangeness. If you’ve no idea what the order of an element is – patience grasshopper (alternatively, hop over to John Armstrong’s excellent The Unapologetic Mathematician which, in addition to inspiring this blog, has a great write up of group theory from a while back.)

Axiom 1 causes some quaint strangeness like the fact that, give quandles Q, P then \forall p \in P: Q \cong Q \oplus \{p\}. Axiom 3 causes strangeness that gives me headaches. It’s also what makes quandles so interesting – axiom 3 is when quandles become nonassociative. Compare the following:

1. (a \cdot b) \cdot c = a \cdot (b \cdot c);

2. (a \vartriangleright b) \vartriangleright c = (a \vartriangleright c) \vartriangleright (b \vartriangleright c).

The first is familiar, it says something is associative – for example (3 + 4) + 5 = 3 + (4 + 5). The second is weird. It says that in quandles, our operation distributes over itself! That’s like saying (3 + 4) + 5 = (3 + 5) + (4 + 5). Ultimately, this is what drew me to quandles – the fact that such a weird structure would crop up very naturally in a number of places.

As always, if you spot any mistakes or would like references on any of the above, just ask. And sorry for the many forward references.

Up next: Examples of Quandles – The trivial quandle on N elements, the Tait-3 quandles, conjugation quandles, and more…

PS. As a side note, today I checked out Baby Rudin 2nd ed. from the science library today. Time permitting (which is a huge if this semester), and in spite of the fact I’m taking Advanced Calc right now, I’m planning on working my way through is – proving his theorems before reading his proofs, doing the problems, and just generally putting some mathematical hair in my beard. (For those of you that don’t know, Rudin’s Principles of Mathematical Analysis is held in quite high esteem by the many people who learned analysis from its dense but rich pages.) Expect updates. Funny, no?

[1] Some history: As far as I know, this connection (specifically, that there’s a presentation of the fundamental group of the knot complement with only conjugation relations – groups under conjugation form quandles as we’ll see later) was first established by David Joyce. While Joyce coined the name quandle and – to my knowledge – first established their topological usage, quandles were considered at least as early as the 1950’s by John Conway (who called them wracks and didn’t impose axiom 3.)

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Introduction

So I suppose I should start with the obligatory “Who the hell are you and why should I read this?” post. My name’s Geoff, I’m currently a junior year (I think… long story that I may or may not tell) undergraduate in Theoretical Math at the University of Akron.

I study math because I legitimately enjoy the subject, hard as that may be to believe if you’re not familiar with upper level mathematics. I also study computer science because I like things like programming, operating systems, and databases. Yes, I run Linux. Beyond that… we’ll see where it goes.

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