GROUPOID C""ALGEBRAS 1 Introduction 2 Definitions and notation

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90 M¼ad¼alina Roxana Buneci4. If (s) = (t), then there is x 2 2 such that s x = t.5. If (s) = (t), then there is 2 1 such that s = t.The groupoids 1; 2 are called Morita equivalent.The notion of Morita equivalence de…ned above is an equivalence relation onlocally compact Hausdor¤ groupoids having open range maps (see [13]).Examples1. Any locally compact groupoid having open range map is a ( ; )-Moritaequivalence (Examples 5.33.1 [14])2. If 1, 2 are locally compact Hausdor¤ groupoids having open range maps,' : 1 ! 2 is an isomorphism and if ' is a homeomorphism then 1 is a( 1 ; 2)-Morita equivalence ( 1 acts to the left on 1 by multiplication and 2acts to the right on 1 by y x = y' (x))(Examples 5.33.2 [14]).3. If G is a locally compact Hausdor¤ transitive groupoid which is second countableand u is a unit in G (0) then G u is a (G; G u u)-Morita equivalence (Theorem2.2A, Theorem 2.2B [13]). More generally, if is a locally compact Hausdor¤groupoid, F is a closed subset of (0) and if the restrictions of r and d to Fare open, then F is a ( ; j F )-Morita equivalence.4. If R is an equivalence relation on a locally compact Hausdor¤ space X, suchthat R as a subset of X X is a closed set, then X implements a Moritaequivalence between the groupoid R (see 5 in Subsection 2.3), and the groupoidX=R (see 2 in Subsection 2.3) (Examples 5.33.5 [14]).De…nition 5. Let A be a C -algebra. A pre-Hilbert A-module is a right A-moduleX (with a compatible C-vector space structure) equipped with a conjugate-bilinearmap (liner in the second variable) h; i A : X X ! A satisfying:1. hx; y ai A= hx; yi A a for all x; y 2 X, a 2 A.2. hx; yi A = hy; xi Afor all x; y 2 X.3. hx; xi A 0 for all x 2 X.4. hx; xi A= 0 only when x = 0.The map h; i Ais called A-valued inner product on X:A left pre-Hilbert A-module is de…ned in the same way, except that X is requiredto be a left A-module, the map A h; i : X X ! A is required to be linear in the…rst variable, and the …rst condition above is replaced by A ha x; yi = a A hx; yifor all x; y 2 X, a 2 A ([15], [21]).******************************************************************************Surveys in Mathematics and its Applications 1 (2006), 71 –98http://www.utgjiu.ro/math/sma
Groupoid C -algebras 91It can be shown that kxk = khx; xik 1 2 de…nes a norm on X. If X is completewith respect to this norm, it is called a Hilbert A-module. If not, all the structurecan be extended to its completion to turn it into a Hilbert A-module.As a normed linear space, a right Hilbert A-module X carries an algebra ofbounded linear transformations. In the following we shall denote by B (X) thecollection of all bounded linear maps T : X ! X which are module maps (this meansthat T (x a) = T (x)a for all x 2 X, a 2 A) and adjointable (this means that thereis another linear bounded operator T on X such that hT (x) ; yi A= hx; T (y)i Aforall x; y 2 X). It is easy to see that if X is Hilbert space and A = C (the space ofcomplex numbers), then B (X) is the algebra of linear bounded operators on X. Thealgebra of linear bounded operators on a Hilbert X has a two-sided closed non-trivialideal K (X) (the compact operators). The analog of that ideal of compact operatorsis given in the case of a right Hilbert A-module X by the closed linear span in B (X)of all the "rank one" transformations on X, i.e. of all transformations of the formx y : X ! X de…ned by x y (z) = x hy; zi Afor all z 2 X, with x; y 2 X. Theclosed linear span in B (X) of all transformations xy (with x; y 2 X) is denoted byK (X), it is called the imprimitivity algebra of X and its elements are called compactoperators on X. If X is a left Hilbert A-module, then imprimitivity algebra K (X)is the closed linear span of all transformations of the form x y : X ! X de…nedby x y (z) = A hz; yi x for all z 2 X, with x; y 2 X.Proposition 6. If A is a C -algebra and if X is a right Hilbert A-module, then B (X)and K (X) are C -algebras, with B (X) equal to the multiplier algebra of K (X).Further, X becomes a left Hilbert C -module over K (X), where the K (X)-valuedinner product is de…ned by the formula:K(X) hx; yi = x y , for all x; y 2 X,and K (X) (X as a left Hilbert C -module over K (X)) is naturally isomorphic to Athough the formulax y ! hx; yi A.De…nition 7. Let A and B be C -algebras. By an (A; B)-equivalence bimodule wemean an A, B -bimodule X equipped with A and B-valued inner products with respectto which X is a right Hilbert B-module and a left Hilbert A-module such that:1. A hx; yi z = x hy; zi Bfor all x; y; z 2 X.2. hax; axi B kak 2 hx; xi Bfor all a 2 A, x 2 X and A hxb; xbi kbk 2 Ahx; xi forall b 2 B, x 2 X.3. hX; Xi Bspans a dense subset of B and A hX; Xi spans a dense subset of A.******************************************************************************Surveys in Mathematics and its Applications 1 (2006), 71 –98http://www.utgjiu.ro/math/sma
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GROUPOID C""ALGEBRAS 1 Introduction 2 Definitions and notation

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