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Symmetry breaking for representations of rank one orthogonal groups. (English) Zbl 1334.22015
Mem. Am. Math. Soc. 1126, v, 110 p. (2015).
The memoir is written by leading experts in representation theory of Lie groups. It contains 16 chapters, including an introduction and an appendix for a total of 112 pages. The main topic is the study of restrictions of representations of real rank one (indefinite) orthogonal groups.
Let $$G$$ be a Lie group and $$G'$$ a subgroup of $$G$$. An important problem is to understand the restriction to $$G'$$ of an irreducible representation $$\pi$$ of $$G$$, also known as symmetry breaking. When $$G$$ is compact this question is well understood. For instance, every representation of $$G$$ is unitary and $$\pi_{\mid_{G'}}$$ is isomorphic to a direct sum of irreducible representations $$\pi'$$ of $$G'$$ with multiplicity $$m(\pi,\pi')$$. Moreover, in the formulation of representation theory of (connected) compact groups in terms of coadjoint orbits, the representation $$\pi$$ is associated with the orbit through the highest weight of $$\pi$$ and $$m(\pi,\pi')$$ can be explicitly described in terms of partition functions. When $$\pi$$ is infinite dimensional and $$G'$$ is non-compact, the restriction $$\pi_{\mid_{G'}}$$ is no longer a direct sum of irreducible representations.
Let $$H$$ be a closed subgroup of $$G$$ and $$(\lambda,V)$$ a finite-dimensional complex representation of $$H$$. One gets a homogeneous vector bundle $${\mathcal V}:=G\times_HV$$ over the homogeneous space $$X:=G/H$$. The space $$C^\infty(X,{\mathcal V})$$ of smooth sections identifies with $$V$$-valued smooth functions on $$G$$ satisfying the equivariance relation: $f(gh)=h^{-1}\cdot f(g)\;\;\; \forall g\in G,h\in H.$ The group $$G$$ acts by left translations on $$C^\infty(X,{\mathcal V})$$. Similarly, if $$H'$$ is a subgroup of $$G'$$ and $$(\nu,W)$$ is a finite dimensional complex representation of $$H'$$, one gets a $$G'$$-module $$C^\infty(Y,{\mathcal W})$$ where $$Y:=G'/H'$$ and $${\mathcal W}:=G\times_HW$$. A continuous $$G'$$-homomorphism $$C^\infty(X,{\mathcal V})\rightarrow C^\infty(Y,{\mathcal W})$$ is called a symmetry breaking operator. The multiplicity $$m(\pi,\pi')$$ is the (possibly infinite) dimension of the space $$H(\lambda,\nu)$$ of symmetry breaking operators. In the paper under review, the authors provide a complete self-contained study and classification of symmetry breaking operators in the case of indefinite orthogonal groups.
From now on, suppose $$G$$ is the indefinite orthogonal group $$O(n+1,1)$$ and $$G'$$ is the subgroup $$O(n,1)$$ realized as follows, $$n\geq 1$$. $$G$$ is the group preserving the quadratic form $$x_0^2+x_1^2+\dots+x_n^2-x_{n+1}^2$$ of signature $$(n+1,1)$$. The subgroup $$G'$$ is the stabilizer of the vector $$e_n=\text{}^{t}(0,\dots,0,1,0)$$ in $$\mathbb R^{n+2}$$. The subgroup $$K:=O(n+1)\times O(1)$$ is a maximal compact subgroup of $$G$$ and $$K':=K\cap G'$$ is a maximal compact subgroup of $$G'$$. Let $${\mathfrak g}={\mathfrak o}(n+1,1)$$ (resp. $${\mathfrak g}'={\mathfrak o}(n,1))$$ be the Lie algebra of $$G$$ (resp. $$G'$$). With the usual notation $$\{E_{i,j}\}_{0\leq i,j\leq n+1}$$ for the canonical basis of $$gl(n+2,{\mathbb R})$$, fix the hyperbolic element $$H:=E_{0,n+1}+E_{n+1,0}$$ in $${\mathfrak g}'$$. Then $$\text{ad}(H)$$, as an endomorphism of $${\mathfrak g}$$, has eigenvalues $$0$$ and $$\pm 1$$. Set $${\mathfrak a}:={\mathbb R}H$$, $${\mathfrak n}_{\pm}:=\text{Ker}(\text{ad}(H)\pm 1)$$, $$A:=\exp({\mathfrak a})$$, $$N_{\pm}:=\exp({\mathfrak n}_{\pm})$$ and $$M:=Z_K({\mathfrak a})\simeq O(n)\times{\mathbb Z}_2$$. Then $$P:=MAN_+$$ is a Langlands decomposition of a minimal parabolic subgroup of $$G$$. Likewise $$P':=M'AN_+'$$ is a Langlands decomposition of a minimal parabolic subgroup of $$G'$$ with $$M':=Z_{K'}({\mathfrak a})\simeq O(n-1)\times{\mathbb Z}_2$$, $$N_{\pm}':=\exp({\mathfrak n}_{\pm}')$$ and $${\mathfrak n}_{\pm}':={\mathfrak n}_{\pm}\cap{\mathfrak g}'$$. The fact that $$M$$ and $$M'$$ are both compact, with $$P$$ and $$P'$$ having the same $$A$$-part, makes the study of the space $$H(\lambda,\nu)$$ of symmetry breaking operators more tractable.
Chapter 2 contains the proof of results about $$G'$$-intertwining operators between irreducible composition factors of spherical principal series representations and symmetry breaking operators for spherical principal series representations of $$G$$ and $$G'$$. Chapter 3 provides a method to study symmetry breaking operators for induced representations by means of their distribution kernels. Chapter 4 contains the realization in the non-compact picture of spherical principal series and a normalization $$\widetilde{{\mathbb T}}_\lambda$$ of the Knapp-Stein intertwining operators $${\mathbb T}_\lambda$$, while Chapter 5 deals with the double coset decompositions $$G'\backslash G/P$$ and $$P'\backslash G/P$$. Chapters 6 to 10 prepare the complete classification of symmetry breaking operators from principal series representations of $$G$$ to those of $$G'$$ contained in Chapter 11, based on some results proved in Chapters 12 and 13. As a byproduct, the authors analyse vector bundles over the anti-de Sitter spaces $$G/G'$$ in Chapter 14, while in Chapter 15 they obtain an explicit construction of the complementary series representations of $$G$$ along with their branching laws. Finally, Chapter 16 is an appendix which collects useful information about Gegenbauer polynomials, (normalized) $$K$$-Bessel functions and Zuckerman derived functor modules $$A_{\mathfrak q}(\lambda)$$.
Reviewer: Salah Mehdi (Metz)

##### MSC:
 22-02 Research exposition (monographs, survey articles) pertaining to topological groups 22E46 Semisimple Lie groups and their representations 22E45 Representations of Lie and linear algebraic groups over real fields: analytic methods 33C80 Connections of hypergeometric functions with groups and algebras, and related topics 53C35 Differential geometry of symmetric spaces
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