Let n be an odd integer greater than 3 and X be the (n+1)-dimensional anti-de Sitter space–time. Binegar and Zierau have constructed in Commun. Math. Phys. 138, 245–258 (1991) a unitary representation H=H+⊕H− of the conformal group SOo(n+1,2) of X. They have shown that this particular representation is, in a certain algebraic sense, a quantization of the union, On+1min=On+1min+∪On+1min−, of the two minimal nilpotent orbits in so(n+1,2) [Commun. Math. Phys. 138, 245–258 (1991)]. (This work is part of the author’s Ph.D. thesis.) In spite of this, it is known how to obtain ℋ from On+1min using a known quantization procedure. One reason for this interest in ℋ is that, in the case n=3, H+ is the representation carried by the one-particle sector of the massless scalar field on the anti-de Sitter space–time X. In this paper, we strengthen this link between the coadjoint orbits On+1min± and H±, by studying the semiclassical limit of the latter. In this way, we clarify their appearance in the massless theory and corroborate the existing evidence that H± is the “correct” quantization of On+1min±. As a preliminary, we show that the projection onto so(n,2)* of On+1min± is the union of the two coadjoint orbits Ono± and Onmin± (Proposition IV.1); one of those is the phase space of the classical massless particle on X. We then show (Theorem VI.1 and Corollary VI.3) that the semiclassical limit of the spectral counting function of the generator of the SO(2) subgroup of SOo(n,2) in the representation H± is dominated by a Weyl term, expressed naturally in terms of the symplectic volume of a compact portion of the classical phase space Ono±. Furthermore, we show (Theorem VI.4) that the highest weight vectors of the representation coincide in the semiclassical limit with the BKW functions constructed starting from Ono±. We show in addition that, even though the orbit method applied to On+1min± does not yield H±, it nevertheless establishes a natural relation between them. Namely, the simple SO(n+1)×S0(2)-modules appearing in H+ are those we obtain if we apply the orbit method to integral SO(n+1)×S0(2)-orbits contained in the projection on (so(n+1)⊕so(2))* of On+1min± (Sec. VII). As a by-product of our analysis, we study the restriction to SOo(n,2) of ℋ (Proposition VIII.1) and we show that the unitary structure on ℋ is exactly a Klein–Gordon scalar product on X (Proposition IX.2).
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