The aim of this short communication is to review some classical results on string topology of manifolds and discuss recent extensions of this theory to orbifolds. In particular, we review the relation between the loop homology of the classiying space of the orbifold and the Hochschild cohomology of dg-ring naturally associated to the orbifold.
IntroductionString topology is the study of the topological properties of to the space of smooth loops LM on a closed and oriented manifold M . In particular algebraic structures defined on the homology of the loop space. The area started with the paper 5 by Chas and Sullivan where they defined an intersection product in the homology of the free loop spacehaving total degree −n.Orbifolds, originally known as V -manifolds by I. Satake, 18 and named this way by W. Thurston, 19 are a generalizations of manifolds, locally they look like the quotient of euclidean space by the action of a finite group. They appear naturally in many areas such as moduli problems, noncommutative geometry and topology. It is natural to ask, What is the string topology of an orbifold? In this survey article we give different answers in the case where the orbifold is a global quotient.The character of this paper is expository and is organized as follows. In section one we review some of the classical theory for manifolds, and state Geometric, Algebraic and Topological Methods for Quantum Field Theory Downloaded from www.worldscientific.com by UNIVERSITY OF BIRMINGHAM LIBRARY -INFORMATION SERVICES on 03/22/15. For personal use only. 332 A. Angelthe results of Chas-Sullivan 5 and Cohen-Jones 7 on the string topology of manifolds. In section two we discuss the necessary background on orbifolds. In section three, we review the construction of 14 of the string topology of quotient orbifolds over the rationals and the construction of 3 of the string topology of quotient orbifolds over the integers. In section four we specialize to the case of [ * /G] and give geometric descriptions of the string topology product on the cohomology H * (LBG).
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