Filed under: Algebra, Math | Tags: algebra, geometry, group theory, linear algebra, MaBloWriMo, Math, topology

Okay, here’s the moment you’ve been waiting for: the proof of the Banach-Tarski Paradox. Here’s what the paradox says:

**Theorem** (Banach-Tarski). *There are a finite number of disjoint subsets of whose union is the unit ball, and such that we can apply an isometry to each of them and wind up with disjoint sets whose union is a pair of unit balls.*

Or “we can cut a unit ball up into a finite number of pieces, rearrange them, and put them back together to make two balls.”

Filed under: Algebra, Math | Tags: algebra, categorical, group theory, MaBloWriMo, Math

Ugh, so, I’ve been really busy today and haven’t had the time to do a Banach-Tarski post. Since I really do want to see MaBloWriMo to the end, I’m going to take a break from the main exposition and quickly introduce something useful. There are a couple major ways of combining two groups into one. The most important one, called the direct product, is analogous to the product of topological spaces. I know this is sort of a wussy post — sorry.

Filed under: Algebra, Math | Tags: algebra, geometry, group theory, MaBloWriMo, Math, topology

Finite-dimensional vector spaces come packed with something extra: an inner product. An **inner product** is a map that multiplies two vectors and gives you a scalar. It’s usually written with a dot, or with angle brackets. For real vector spaces, we define it to be a map with the following properties:

- Symmetry:
- Bilinearity: , where are scalars and is another vector, and the same for the second coordinate
- Positive-definiteness: , and it is only equal to when .

(I’m going to stop using boldface for vectors, since it’s usually clear what’s a vector and what’s not.) One of the uses of an inner product is to define the **length** of a vector: just set . This is only if is, and otherwise it’s always real and positive because the inner product is positive definite. Another use is to define the **angle** between two nonzero vectors: set . In particular, is right iff . In this case, we say and are **orthogonal**.

In Euclidean space, the inner product is the **dot product**: . This is primarily what we’re concerned with today, so we’ll return to abstract inner products another day.

Filed under: Algebra, Math | Tags: algebra, graph theory, group theory, MaBloWriMo, Math, pretty pictures

Today we’re going to take the abstract group machinery we’ve been building up, and unleash it on some sets. When mathematicians say that “groups describe symmetry,” this is exactly what they’re talking about. Say we have a set, and some “symmetries” of that set. Pretty much any definition of “symmetry” will take it to be a bijection on the set, and we moreover expect to be able to undo symmetries, to compose them (associatively), and to use the identity map as a symmetry. These heuristics are just informal versions of the group axioms! The element we’ve been leaving out so far, though, is the set itself on which the symmetries are founded. We say that this symmetry group **acts on** this set. Below, let’s make this formal.

Last time we talked about the cosets of a subgroup. We showed that for a normal subgroup, the left and right cosets coincide, and thus you could multiply any two cosets to get a third coset, defining a group operation on the set of equivalence classes of cosets, or quotient group. We proved the powerful First Isomorphism Theorem, which characterized normal subgroups as the kernels of homomorphisms and showed that the quotient of a homomorphism’s domain with its kernel was isomorphic to its image.

Today, we’re going to introduce a new “type” of group and use the First Isomorphism Theorem to develop a new way of representing groups.

Today I have more of a specific idea of what I want to talk about, since we’ve got some background out of the way. We’re going to define quotient groups and look at some examples. The quotient operation is dual to the subgroup operation: a quotient group is, effectively, what you get when you “trivialize” a subgroup. This general idea adapts itself to many other mathematical structures, like, as we’ll see, topological spaces. For groups, however, it’s only possible to do this for certain kinds of subgroups, and so a little background is needed. Let’s get started!

Filed under: Algebra, Math | Tags: algebra, combinatorics, group theory, MaBloWriMo, Math

Like yesterday, today is pretty much just going to be me talking about groups until I feel like stopping. I’m hoping to osmose to you some of the ways in which groups “work” and that you can deal with them. Pretty soon, we should have the background ready for one of the neatest paradoxes/counterexamples in math — the Banach-Tarski Paradox!