# MA667 Presentations of Groups

**Lecturer:** Derek Holt

**Term:** 2

**Status for Mathematics students:** List C . This module is suitable for Third Year MMath students

**Commitment:** 30 one-hour lectures

**Assessment:** Three-hour written examination (100%).

**Prerequisites:** MA251 Algebra I and MA249 Algebra II

**Leads To: **Postgraduate work in Group Theory

**Content**: This module is about groups that are defined by means of a presentation in terms of generators and relations. This means that a set of generators *X* is given for the group *G*, and a set of defining relations *R*. Defining relations are equations involving the generators and their inverses, which are required to hold in *G*. Then *G* is defined to be essentially the largest group that is generated by a set *X* for which the defining relations hold. For example, the dihedral group of order 6 could be defined as the group with generating set $ X = \{x,y\} $and relations $ R= \{x^3=1, y^2=1, yxy=x^{-1} \} $.

This method of defining a group has the advantage that it is often the most concise description of the group possible. Furthermore, groups arising from algebraic topology often appear naturally in this form. The disadvantage of the method is that it can be very difficult (and even theoretically impossible in some cases) to derive important properties of a group *G* that is given only by a presentation, such as whether it is finite, abelian, etc., However, as a result of the frequency with which group presentations crop up in other branches of mathematics, the development of techniques for finding out information about these groups has become a major branch of mathematical research.

In this module, we shall be developing the basic theory of group presentations, and looking at some particular techniques for analysing them. We start with free groups (groups with no defining relations) and prove a fundamental theorem of Schreier, that a subgroup of a free group is itself free. We then move on to presentations in general, and look at lots of examples. In the later part of the module, we shall be looking at some algorithmic methods for studying group presentations, including the Todd-Coxeter algorithm for calculating the index of a subgroup *H* of finite index in *G*, and the Reidemeister-Schreier method for calculating a presentation of *H*. (These algorithms are highly suitable for computer implementation, although we will not be studying that aspect of them in detail in this course.)

**Aims**: To illustrate the important general notion of definition of an algebraic structure by generators and defining relations in the context of group theory.

To develop some examples of the use of algorithmic methods in pure mathematics.

**Objectives**: To give a mathematically precise but comprehensible treatment of the definition of a group by generators and relations, and to teach students how to start extracting elementary information about the group from its presentation.

To teach students how to carry out the Todd-Coxeter coset enumeration algorithm by hand in simple examples, and how to compute presentations of subgroups of groups.

**Books**:

D.L. Johnson, *Presentations of Groups* (Second Edition), LMS Student Texts 15 C.U.P. 1997, Chapters 1,2,4,5,8,9.