# B3.5 Topology and Groups - Material for the year 2019-2020

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Part A Topology is essential and Group Theory is recommended.

16 lectures

### Assessment type:

- Written Examination

This course introduces the important link between topology and group theory. On the one hand, associated to each space, there is a group, known as its fundamental group. This can be used to solve topological problems using algebraic methods. On the other hand, many results about groups are best proved and understood using topology. For example, presentations of groups, where the group is defined using generators and relations, have a topological interpretation. The endpoint of the course is the Nielsen-Shreier Theorem, an important, purely algebraic result, which is proved using topological techniques.

Students will develop a sound understanding of simplicial complexes, cell complexes and their fundamental groups. They will be able to use algebraic methods to analyse topological spaces. They will also be able to address questions about groups using topological techniques.

Homotopic mappings, homotopy equivalence. Simplicial complexes. Simplicial approximation theorem.

The fundamental group of a space. The fundamental group of a circle. Application: the fundamental theorem of algebra. The fundamental groups of spheres.

Free groups. Existence and uniqueness of reduced representatives of group elements. The fundamental group of a graph.

Groups defined by generators and relations (with examples). Tietze transformations.

The free product of two groups. Amalgamated free products.

The Seifert-van Kampen Theorem.

Cell complexes. The fundamental group of a cell complex (with examples). The realization of any finitely presented group as the fundamental group of a finite cell complex.

Covering spaces. Liftings of paths and homotopies. A covering map induces an injection between fundamental groups. The use of covering spaces to determine fundamental groups: the circle again, and real projective $n$-space. The correspondence between covering spaces and subgroups of the fundamental group. Regular covering spaces and normal subgroups.

Cayley graphs of a group. The relationship between the universal cover of a cell complex, and the Cayley graph of its fundamental group. The Cayley 2-complex of a group.

The Nielsen-Schreier Theorem (every subgroup of a finitely generated free group is free) proved using covering spaces.

- John Stillwell,
*Classical Topology and Combinatorial Group Theory*(Springer-Verlag, 1993).

- D. Cohen,
*Combinatorial Group Theory: A Topological Approach*, Student Texts 14 (London Mathematical Society, 1989), Chapters 1-7. - A. Hatcher,
*Algebraic Topology*(CUP, 2001), Chapter. 1. - M. Hall, Jr,
*The Theory of Groups*(Macmillan, 1959), Chapters. 1-7, 12, 17 . - D. L. Johnson,
*Presentations of Groups*, Student Texts 15 (Second Edition, London Mathematical Society, Cambridge University Press, 1997). Chapters. 1-5, 10,13. - W. Magnus, A. Karrass, and D. Solitar,
*Combinatorial Group Theory*(Dover Publications, 1976). Chapters. 1-4.