Research Article

Dynamical systems of type (m,n) and their C^*-algebras

PERE ARA^a1, RUY EXEL^a2 and TAKESHI KATSURA^a3

^a1 Departament de Matemátiques, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Spain (email: para@mat.uab.cat)

^a2 Departamento de Matemática, Universidade Federal de Santa Catarina, 88010-970 Florianópolis SC, Brazil (email: exel@mtm.ufsc.br)

^a3 Department of Mathematics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama, 223-8522, Japan (email: katsura@math.keio.ac.jp)

Abstract

Given positive integers n and m, we consider dynamical systems in which (the disjoint union of) n copies of a topological space is homeomorphic to m copies of that same space. The universal such system is shown to arise naturally from the study of a C*-algebra denoted by 𝒪 _m,n , which in turn is obtained as a quotient of the well-known Leavitt C*-algebra L _m,n , a process meant to transform the generating set of partial isometries of L _m,n into a tame set. Describing 𝒪 _m,n as the crossed product of the universal (m,n) -dynamical system by a partial action of the free group 𝔽 _m+n , we show that 𝒪 _m,n is not exact when n and m are both greater than or equal to 2, but the corresponding reduced crossed product, denoted by 𝒪 ^r _m,n , is shown to be exact and non-nuclear. Still under the assumption that m,n≥2 , we prove that the partial action of 𝔽 _m+n is topologically free and that 𝒪 ^r _m,n satisfies property (SP) (small projections). We also show that 𝒪 ^r _m,n admits no finite-dimensional representations. The techniques developed to treat this system include several new results pertaining to the theory of Fell bundles over discrete groups.

(Received November 15 2011)

(Revised April 05 2012)

(Online publication 2012)