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单词 GroupoidCconvolutionAlgebras
释义

groupoid C*-convolution algebras


0.1 Introduction: Background and definition of the groupoid C*–convolution algebra

Jean Renault introduced in ref. [6] the C*–algebraPlanetmathPlanetmath of a locally compact groupoidPlanetmathPlanetmath G as follows: the space of continuous functionsMathworldPlanetmathPlanetmath with compact support on a groupoidPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath 𝖦 is made into a *-algebra whose multiplication is the convolution, and that is also endowed with the smallest C*–norm which makes its representations continuous, as shown in ref.[3]. Furthermore, for this convolution to be defined, one needs also to have a Haar systemPlanetmathPlanetmath (http://planetmath.org/GroupoidRepresentationsInducedByMeasure)associated to the locally compact groupoids (http://planetmath.org/LocallyCompactGroupoids) 𝖦that are then called measured groupoids because they are endowed with an associated Haar system which involves the concept of measure, as introduced in ref. [1] by P. Hahn.

With these concepts one can now sum up the definition (or construction) of the groupoid C*-convolution algebra, or http://www.utgjiu.ro/math/mbuneci/preprint/p0024.pdfgroupoid C*-algebra, as follows.

Definition 0.1.

a groupoid C*–convolution algebra, GCA, is defined for measured groupoidsas a *–algebra with “*” being defined by convolution so that it has a smallest C*–norm which makes its representations continuous.

Remark 0.1.

One can also produce a functorial construction of GCA that has additional interesting properties.

Next we recall a result due to P. Hahn [2] which shows how groupoid representationsPlanetmathPlanetmathPlanetmathPlanetmath relate toinduced *-algebra representations and also how–under certain conditions– the former can be derived fromthe appropriate *-algebra representations.

Theorem 0.1.

(source: ref. [2]). Any representation of a groupoid (G,C) with Haar measure (ν,μ) in a separable Hilbert space H induces a *-algebra representation fXf of the associatedgroupoid algebra Π(G,ν) in L2(UG,μ,H) with the following properties:

(1) For any l,mH , one has that |<Xf(ul),(um)>|fllm and

(2) Mr(α)Xf=Xfαr, where

Mr:L(U𝖦,μL[L2(U𝖦,μ,], with

Mr(α)j=αj.

Conversely, any *- algebra representation with the above two properties induces a groupoid representation, X, as follows:

<Xf,j,k>=f(x)[X(x)j(d(x)),k(r(x))dν(x)].(viz. p. 50 of ref. [2]).

Furthermore, according to Seda (ref. [10, 11]), the continuity of a Haar system is equivalentMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath to the continuity of the convolution productPlanetmathPlanetmath f*g for any pair f, g of continuous functions with compact support. One may thus conjecture that similar results could be obtained for functions with locally compact supportMathworldPlanetmathPlanetmathPlanetmath in dealing with convolution products of either locally compact groupoids or quantum groupoidsPlanetmathPlanetmathPlanetmath. Seda’s result also implies that the convolution algebra Cc(𝒢) of a groupoid 𝒢 is closed with respect to convolution if and only if the fixed Haar system associated with the measured groupoid 𝒢 is continuous (see ref. [3]).

Thus, in the case of groupoid algebras of transitive groupoids, it was shown in [3] that any representation of a measured groupoid (𝒢,[νu𝑑λ~(u)]=[λ]) on a separable Hilbert space induces a non-degenerate *-representation fXf of the associated groupoid algebraΠ(𝒢,ν,λ~) with properties formally similar to (1) and (2) above.Moreover, as in the case of groups, there is a correspondence between the unitary representationsMathworldPlanetmath of a groupoid and its associated C*-convolution algebra representations (p. 182 of [3]), the latter involving however fiber bundlesMathworldPlanetmath of Hilbert spacesMathworldPlanetmath instead of single Hilbert spaces.

References

  • 1 P. Hahn: Haar measure for measure groupoids., Trans. Amer. Math. Soc. 242: 1–33(1978).
  • 2 P. Hahn: The regular representations of measure groupoids., Trans. Amer. Math. Soc. 242:35–72(1978).Theorem 3.4 on p. 50.
  • 3 M. R. Buneci. Groupoid Representations, Ed. Mirton: Timishoara (2003).
  • 4 M.R. Buneci. 2006.,http://www.utgjiu.ro/math/mbuneci/preprint/p0024.pdfGroupoid C*-Algebras.,Surveys in Mathematics and its Applications, Volume 1: 71–98.
  • 5 M. R. Buneci. IsomorphicPlanetmathPlanetmathPlanetmath groupoid C*-algebras associated withdifferent Haar systems., New York J. Math., 11 (2005):225–245.
  • 6 J. Renault. A groupoid approach to C*-algebras, Lecture Notes in Math., 793, Springer,Berlin, (1980).
  • 7 J. Renault. 1997. The Fourier Algebra of a Measured Groupoid and Its Multipliers,Journal of Functional AnalysisMathworldPlanetmath, 145, Number 2, April 1997, pp. 455–490.
  • 8 A. K. Seda: Haar measures for groupoids, Proc. Roy. Irish Acad.Sect. A 76 No. 5, 25–36 (1976).
  • 9 A. K. Seda: Banach bundles of continuous functions and an integralrepresentation theorem, Trans. Amer. Math. Soc. 270 No.1 : 327-332(1982).
  • 10 A. K. Seda: On the Continuity of Haar measures on topological groupoids, Proc. Amer Math. Soc. 96: 115–120 (1986).
  • 11 A. K. Seda. 2008. Personal communication, and also Seda (1986, on p.116).
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