Citation:
Mathematische Nachrichten, 2002, vol. 239-240, n. 1, p. 11-27

ISSN:
0025-584X (Print) 1522-2616 (Online)

DOI:
10.1002/1522-2616(200206)239:1<11

Sponsor:
The second author (F.Ll.) expresses his gratitude to Sergio Doplicher for his hospitality at the ‘Dipartamento di Matematica dell’Università di Roma "La Sapienza" in october’99. The visit was supported by a EU TMR network "Implementation of concept and methods from Non–Commutative Geometry to Operator Algebras and its applications”, contract no. ERB FMRXCT 96-0073.

Given a C*-algebra $A$, a discrete abelian group $X$ and a homomorphism $\Theta: X\to$ Out$A$ defining the dual action group $\Gamma\subset$ aut$A$, the paper contains results on existence and characterization of Hilbert $\{A,\Gamma\}$, where the action is givGiven a C*-algebra $A$, a discrete abelian group $X$ and a homomorphism $\Theta: X\to$ Out$A$ defining the dual action group $\Gamma\subset$ aut$A$, the paper contains results on existence and characterization of Hilbert $\{A,\Gamma\}$, where the action is given by $\hat{X}$. They are stated at the (abstract) C*-level and can therefore be considered as a refinement of the extension results given for von Neumann algebras for example by Jones [Mem.Am.Math.Soc. 28 Nr 237 (1980)] or Sutherland [Publ.Res.Inst.Math.Sci. 16 (1980) 135]. A Hilbert extension exists iff there is a generalized 2-cocycle. These results generalize those in [Commun.Math.Phys. 15 (1969) 173], which are formulated in the context of superselection theory, where it is assumed that the algebra $A$ has a trivial center, i.e. $Z=C1$. In particular the well-known ``outer characterization'' of the second cohomology $H^2(X,{\cal U}(Z),\alpha_X)$ can be reformulated: there is a bijection to the set of all $A$-module isomorphy classes of Hilbert extensions. Finally, a Hilbert space representation (due to Sutherland in the von Neumann case) is mentioned. The C*-norm of the Hilbert extension is expressed in terms of the norm of this representation and it is linked to the so-called regular representation appearing in superselection theory.[+][-]