The notions of reflection, symmetry, and positivity from quantum field theory are shown to induce a duality operation for a general class of unitary representations of Lie groups. The semisimple Lie groups which have this c-duality are identified and they are placed in the context of Harish-Chandra's legacy for the unitary representations program. Our paper begins with a discussion of path space measures, which is the setting where reflection positivity (Osterwalder-Schrader) was first identified as a useful tool of analysis.Le plus court chemin entre deux vérités dans le domaine réel passe par le domaine complexe.
-Jacques HadamardBoth authors are supported in part by the U.S. National Science Foundation. The second author was supported by LEQSF grant (1996-99)-RD-A-12. 1 UNITARY REPRESENTATIONS AND O-S DUALITY 3of P n by passing over to imaginary time. This idea was used in the paper by J. Fröhlich, K. Osterwalder, and E. Seiler in [15], see also [50], to construct quantum field theoretical systems using Euclidean field theory. In this paper we will give some general constructions and ideas related to this problem in the context of the applications (i)-(v) mentioned above, and work out some simple examples.In [95] R. Schrader used this idea to construct, from a complementary series representation of SL(2n, C), a unitary representation of the group SU (n, n) × SU (n, n). In that paper the similarities to the Yang-Baxter relation were also discussed, a theme that we will leave out in this exposition. What was missing in R. Schrader's paper was the identification of the resulting representations and a general procedure how to construct those representations. We will see that we can do this for all simple Lie groups where the associated Riemannian symmetric space G/K is a tube domain, and the that the duality works between complementary series representations and highest weight representations.In general this problem can be formulated in terms of c-duality of Lie groups and the analytic continuation of unitary representations from one real form to another. (See [33] for these terms.) The representations that show up in the case of semisimple groups are generalized principal series representations on the one side and highest weight representations on the other. The symmetric spaces are the causal symmetric spaces, and the duality is between non-compactly causal symmetric spaces and compactly causal symmetric spaces. The latter correspond bijectively to real forms of bounded symmetric domains. Therefore both the geometry and the representations are closely related to the work of Harish-Chandra on bounded symmetric domains and the holomorphic discrete series [21,22,23]. But the ideas are also related to the work of I. Segal and S. Paneitz through the notion of causality and invariant cones, [96,86,87]. A more complete exposure can be found in the joint paper by the coauthors: Unitary Representations of Lie Groups with Reflection Symmetry, [46].There are other interesting and related questions, problems, and directions...