# C3.5 Lie Groups - Material for the year 2020-2021

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*ASO: Group Theory*, *A5: Topology* and *ASO: Introduction to Manifolds* are all useful but not essential. It would be desirable to have seen notions of derivative of maps from **R^n** to **R^m**, inverse and implicit function theorems, and submanifolds of **R^n**. Acquaintance with the notion of an abstract manifold would be helpful but not really necessary.

16 lectures.

### Assessment type:

- Written Examination

The theory of Lie Groups is one of the most beautiful developments of pure mathematics in the twentieth century, with many applications to geometry, theoretical physics and mechanics. The subject is an interplay between geometry, analysis and algebra. Lie groups are groups which are simultaneously manifolds, that is geometric objects where the notion of differentiability makes sense, and the group multiplication and inversion are differentiable maps. The majority of examples of Lie groups are the familiar groups of matrices. The course does not require knowledge of differential geometry: the basic tools needed will be covered within the course.

Students will have learnt the fundamental relationship between a Lie group and its Lie algebra, and the basics of representation theory for compact Lie groups. This will include a firm understanding of maximal tori and the Weyl group, and their role for representations.

Brief introduction to manifolds. Classical Lie groups. Left-invariant vector fields, Lie algebra of a Lie group. One-parameter subgroups, exponential map. Homomorphisms of Lie groups and Lie algebras. Ad and ad. Compact connected abelian Lie groups are tori. The Campbell-Baker-Hausdorff series (statement only).

Lie subgroups. Definition of embedded submanifolds. A subgroup is an embedded Lie subgroup if and only if it is closed. Continuous homomorphisms of Lie groups are smooth. Correspondence between Lie subalgebras and Lie subgroups (proved assuming the Frobenius theorem). Correspondence between Lie group homomorphisms and Lie algebra homomorphisms. Ado's theorem (statement only), Lie's third theorem.

Basics of representation theory: sums and tensor products of representations, irreducibility, Schur's lemma. Compact Lie groups: left-invariant integration, complete reducibility. Representations of the circle and of tori. Characters, orthogonality relations. Peter-Weyl theorem (statement only).

Maximal tori. Roots. Conjugates of a maximal torus cover a compact connected Lie group. Weyl group. Reflections. Weyl group of *U(n)*. Representations of a compact connected Lie group are the Weyl-invariant representations of a maximal torus (proof of inclusion only). Representation ring of *T&subn;* and *U(n)*.

Killing form. Remarks about the classification of compact Lie groups.

- J. F. Adams,
*Lectures on Lie Groups*(University of Chicago Press, 1982). - T. Bröcker and T. tom Dieck,
*Representations of Compact Lie Groups*(Graduate Texts in Mathematics, Springer, 1985).

- R. Carter, G. Segal and I. MacDonald,
*Lectures on Lie Groups and Lie Algebras*(LMS Student Texts, Cambridge, 1995). - W. Fulton, J. Harris,
*Representation Theory: A First Course*(Graduate Texts in Mathematics, Springer, 1991). - F. W. Warner,
*Foundations of Differentiable Manifolds and Lie Groups*(Graduate Texts in Mathematics, 1983).