# Tag Archives: groups

## Free Groups and Tiling

Introduction Consider the following simple problem. Prove that the shape on the left cannot be completely tiled by 20 polygons of the types shown on the right. The solution is rather simple: colour the shape in the following manner. This … Continue reading

## Quick Guide to Character Theory (I): Foundation

Character theory is one of the most beautiful topics in undergraduate mathematics; the objective is to study the structure of a finite group G by letting it act on vector spaces. Earlier, we had already seen some interesting results (e.g. proof … Continue reading

## Topology: More on Algebra and Topology

We’ve arrived at the domain where topology meets algebra. Thus we have to proceed carefully to ensure that the topology of our algebraic constructions are well-behaved. Let’s look at topological groups again. Our first task is to show that the … Continue reading

## Topology: Topological Groups

This article assumes you know some basic group theory. The motivation here is to consider groups whose underlying operations are continuous with respect to its topology. Definition. A topological group G is a group with an underlying topology such that: the … Continue reading

## Introduction to Ring Theory (6)

Let’s keep stock of what we’ve covered so far for ring theory, and compare it to the case of groups. There are loads of parallels between the two cases. G is a group R is a ring. Abelian groups. Commutative … Continue reading

## Casual Introduction to Group Theory (5)

Normal Subgroups and Group Quotients [ This corresponds to approximately chapter V of the old blog. ] We’ve already seen that if H ≤ G is a subgroup, then G is a disjoint union of (left) cosets of H in G. We’d like to use the set … Continue reading

## Casual Introduction to Group Theory (4)

Cosets and Lagrange’s Theorem [ This post approximately corresponds to chapter IV from the old group theory blog. ] The main theorem in this post is Lagrange’s theorem: if H ≤ G is a subgroup then |H| divides |G|. But first, let’s consider … Continue reading

## Casual Introduction to Group Theory (3)

Subgroups [ This article approximately corresponds to chapter III of the group theory blog. ] Let G be a group under operation *. If H is a subset of G, we wish to turn H into a group by inheriting the operation from G. Clearly, … Continue reading