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Locally compact quantum groups. A von Neumann algebra approach. (English) Zbl 1312.46055
Summary: In this paper, we give an alternative approach to the theory of locally compact quantum groups, as developed by Kustermans and Vaes. We start with a von Neumann algebra and a comultiplication on this von Neumann algebra. We assume that there exist faithful left and right Haar weights. Then we develop the theory within this von Neumann algebra setting. In [S. Vaes and J. Kustermans, Math. Scand. 92, No. 1, 68–92 (2003; Zbl 1034.46067)] locally compact quantum groups are also studied in the von Neumann algebraic context. This approach is independent of the original $$C^*$$-algebraic approach in the sense that the earlier results are not used. However, this paper is not really independent because for many proofs, the reader is referred to the original paper where the $$C^*$$-version is developed. In this paper, we give a completely self-contained approach. Moreover, at various points, we do things differently. We have a different treatment of the antipode. It is similar to the original treatment in [J. Kustermans and S. Vaes, Ann. Sci. Éc. Norm. Supér. (4) 33, No. 6, 837–934 (2000; Zbl 1034.46508)]. But together with the fact that we work in the von Neumann algebra framework, it allows us to use an idea from S. Stratila et al. [Rev. Roum. Math. Pures Appl. 21, 1411–1449 (1976; Zbl 0367.46060)] to obtain the uniqueness of the Haar weights in an early stage. We take advantage of this fact when deriving the other main results in the theory. We also give a slightly different approach to duality. Finally, we collect, in a systematic way, several important formulas. In an appendix, we indicate very briefly how the $$C^*$$-approach and the von Neumann algebra approach eventually yield the same objects.
The passage from the von Neumann algebra setting to the $$C^*$$-algebra setting is more or less standard. For the other direction, we use a new method. It is based on the observation that the Haar weights on the $$C^*$$-algebra extend to weights on the double dual with central support and that all these supports are the same. Of course, we get the von Neumann algebra by cutting down the double dual with this unique support projection in the center. All together, we see that there are many advantages when we develop the theory of locally compact quantum groups in the von Neumann algebra framework, rather than in the $$C^*$$-algebra framework. It is not only simpler, the theory of weights on von Neumann algebras is better known and one needs very little to go from the $$C^*$$-algebras to the von Neumann algebras. Moreover, in many cases when constructing examples, the von Neumann algebra with the coproduct is constructed from the very beginning and the Haar weights are constructed as weights on this von Neumann algebra (using left Hilbert algebra theory).
This paper is written in a concise way. In many cases, only indications for the proofs of the results are given. This information should be enough to see that these results are correct. We will give more details in a forthcoming paper, which will be expository, aimed at non-specialists. See also [A. Van Daele, “Locally compact quantum groups: the von Neumann algebra versus the $$C^*$$-algebra approach”, Bull. Kerala Math. Assoc. 2005, Special Issue, 153–177 (2007)] for an ‘expanded’ version of the appendix.

##### MSC:
 46L10 General theory of von Neumann algebras 43A99 Abstract harmonic analysis 17B37 Quantum groups (quantized enveloping algebras) and related deformations 16T20 Ring-theoretic aspects of quantum groups
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