# B2.1 Introduction to Representation Theory - Material for the year 2020-2021

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2020-2021
Lecturer(s):
Prof. Konstantin Ardakov
General Prerequisites:

Rings and Modules is essential. Group Theory is recommended.

Course Term:
Michaelmas
Course Lecture Information:

16 lectures

Course Weight:
1.00 unit(s)
Course Level:
H

### Assessment type:

Course Overview:

This course gives an introduction to the representation theory of finite groups and finite dimensional algebras. Representation theory is a fundamental tool for studying symmetry by means of linear algebra: it is studied in a way in which a given group or algebra may act on vector spaces, giving rise to the notion of a representation.

A large part of the course will deal with the structure theory of semisimple algebras and their modules (representations). We will prove the Jordan-Hölder Theorem for modules. Moreover, we will prove that any finite-dimensional semisimple algebra is isomorphic to a product of matrix rings (Wedderburn's Theorem over ${\mathbb C}\,$).

In the later part of the course we apply the developed material to group algebras, and classify when group algebras are semisimple (Maschke's Theorem). All of this material will be applied to the study of characters and representations of finite groups.

Learning Outcomes:

They will know in particular simple modules and semisimple algebras and they will be familiar with examples. They will appreciate important results in the course such as the Jordan-Hölder Theorem, Schur's Lemma, and the Wedderburn Theorem. They will be familiar with the classification of semisimple algebras over $\mathbb{C}$ and be able to apply this to representations and characters of finite groups.

Course Synopsis:

Noncommutative rings, one- and two-sided ideals. Associative algebras (over fields). Main examples: matrix algebras, polynomial rings and quotients of polynomial rings. Group algebras, representations of groups.

Modules and their relationship with representations. Simple and semisimple modules, composition series of a module, Jordan-Hölder Theorem. Semisimple algebras. Schur's Lemma, the Wedderburn Theorem, Maschke's Theorem. Characters of complex representations. Orthogonality relations, finding character tables. Tensor product of modules. Induction and restriction of representations. Application: Burnside's $p^aq^b$ Theorem.

1. K. Erdmann, T. Holm Algebras and Representation Theory, Springer Undergraduate Mathematical Series (2018), ISSN 1615-2085
2. G. D. James and M. Liebeck, Representations and Characters of Finite Groups (2nd edition, Cambridge University Press, 2001).
1. J. L. Alperin and R. B. Bell, Groups and Representations, Graduate Texts in Mathematics 162 (Springer-Verlag, 1995).
2. P. M. Cohn, Classic Algebra (Wiley & Sons, 2000). (Several books by this author available.)
3. C. W. Curtis, and I. Reiner, Representation Theory of Finite Groups and Associative Algebras (Wiley & Sons, 1962).
4. L. Dornhoff, Group Representation Theory (Marcel Dekker Inc., New York, 1972).
5. I. M. Isaacs, Character Theory of Finite Groups (AMS Chelsea Publishing, American Mathematical Society, Providence, Rhode Island, 2006).
6. J.-P. Serre, Linear Representations of Finite Groups, Graduate Texts in Mathematics 42 (Springer-Verlag, 1977).
7. P. Etingof, Introduction to representation theory (Online course notes, MIT 2011).