An introduction to noncommutative topology Francesca Arici Mathematical Institute, Leiden University, P.O. Box 9512, 2300 RA Leiden, the Netherlands E-mail address:
[email protected] This work was partially funded by the Netherlands Organisation of Scientific Research (NWO) under the VENI grant 016.192.237. Abstract. C∗{algebras provide an elegant setting for many problems in mathematics and physics. In view of Gelfand duality, their study is often referred to as noncommutative topology: general noncommutative C∗{algebras are interpreted as noncommutative spaces. These form an established research field within mathematics, with applications to quantum theory and other areas where deformations play a role. Many classical geometric and topological concepts can be translated into operator algebraic terms, leading to the so-called noncommutative geometry (NCG) dictionary. These lectures aim to provide the participants with the tools to understand, consult and use the NCG dictionary, covering the basic theory of C∗-algebra and their modules. Focus will be given on examples, especially those that come from deformation theory and quantisation. Contents Chapter 1. An introduction to commutative C∗-algebras5 Motivation 5 1. Trading spaces for algebras5 Chapter 2. Noncommutative C∗-algebas, representations, and the GNS construction 19 1. Representation of C∗-algebras 19 2. States and the GNS construction 22 3. Notable C∗-algebras 25 Chapter 3. Modules as bundles 29 1. Modules and fiber bundles 29 2. Line bundles, Self-Morita equivalence bimodules, and the Picard group 34 Chapter 4. Cuntz{Pimsner algebras and circle bundles 37 1. Pimsner algebras 37 2. Circle actions 39 Bibliography 41 3 CHAPTER 1 An introduction to commutative C∗-algebras Motivation Over the past decades, the term noncommutative topology has come to indicate the study of C∗-algebras.