arXiv subjects

Jan Frahm

Publications and source records attributed to Jan Frahm.

At least 19 recordsLinked to original sources

arXiv2026

Exceptional theta correspondences via Plancherel formulas for rank one symmetric spaces

We consider the minimal representation of (a finite cover of) the conformal group of a simple split Jordan algebra over $\mathbb{R}$ or $\mathbb{C}$, whenever it exists. The conformal group contains a natural dual pair $G\times G'$, where $G$ is essentially the automorphism group of the Jordan algebra and $G'$ is either $\operatorname{PSL}(2,\mathbb{R})$, $\operatorname{PGL}(2,\mathbb{R})$ or $\operatorname{PGL}(2,\mathbb{C})$. The groups $G$ that arise in this way include the complex exceptional group of type $F_4$ as well as its compact and split real form. We explicitly determine the direct integral decomposition of the minimal representation restricted to the corresponding cover of $G\times G'$. This yields a one-to-one correspondence between certain representations of $G$ and (a finite cover of) $G'$. The representations of $G$ that occur in this correspondence are in the support of the Plancherel measure for a rank one symmetric space for $G$, and the proof makes use of the corresponding Plancherel formula.

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arXiv2026

On the degenerate principal series of $G_{2(2)}$ induced from a Heisenberg parabolic subgroup

We study degenerate principal series representations of the split real group $G_{2(2)}$ induced from a character of a maximal parabolic subgroup whose unipotent radical is a Heisenberg group. Using the Lie algebra action on the space of $K$-finite vectors, we find the points of reducibility and the complementary series. The minimal representation and a limit of discrete series are identified as kernel of the corresponding Knapp-Stein intertwining operator. Moreover, we show that some quaternionic discrete series representations occur as the subrepresentation on which the family of intertwining operators vanishes of order two.

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arXiv2025

Branching laws for Stein's complementary series and Speh representations of $\operatorname{GL}(2n,\mathbb{R})$

We obtain the explicit direct integral decomposition of Stein's complementary series representations and Speh representations of $\operatorname{GL}(2n,\mathbb{R})$ when restricted to the subgroup $\operatorname{GL}(2n-1, \mathbb{R})$. The decomposition is a direct integral of unitarily induced representations from a maximal parabolic subgroup of $\operatorname{GL}(2n-1, \mathbb{R})$ with Levi factor $\operatorname{GL}(2n-2, \mathbb{R})\times\operatorname{GL}(1, \mathbb{R})$, where the induction data consists of a complementary series or Speh representation of the factor $\operatorname{GL}(2n-2, \mathbb{R})$ with the same parameter as the one of $\operatorname{GL}(2n, \mathbb{R})$ and a character of $\operatorname{GL}(1, \mathbb{R})$. These results are in line with the theory of adduced representations. The main tools in the proof are two families of symmetry breaking operators between degenerate series representations of $\operatorname{GL}(2n, \mathbb{R})$ and $\operatorname{GL}(2n-1, \mathbb{R})$ whose meromorphic properties are studied in great detail.

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arXiv2024

Explicit Hilbert spaces for the unitary dual of rank one orthogonal groups and applications

We realize all irreducible unitary representations of the group $\mathrm{SO}_0(n+1,1)$ on explicit Hilbert spaces of vector-valued $L^2$-functions on $\mathbb{R}^n\setminus\{0\}$. The key ingredient in our construction is an explicit expression for the standard Knapp-Stein intertwining operators between arbitrary principal series representations in terms of the Euclidean Fourier transform on a maximal unipotent subgroup isomorphic to $\mathbb{R}^n$. As an application, we describe the space of Whittaker vectors on all irreducible Casselman-Wallach representations. Moreover, the new realizations of the irreducible unitary representations immediately reveal their decomposition into irreducible representations of a parabolic subgroup, thus providing a simple proof of a recent result of Liu-Oshima-Yu.

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arXiv2024

Construction and analysis of symmetry breaking operators for the pair $(\operatorname{GL}(n+1,\mathbb{R}),\operatorname{GL}(n,\mathbb{R}))$

The pair of real reductive groups $(G,H)=(\operatorname{GL}(n+1,\mathbb{R}),\operatorname{GL}(n,\mathbb{R}))$ is a strong Gelfand pair, i.e. the multiplicities $\dim\operatorname{Hom}_H(\pi|_H,\tau)$ are either $0$ or $1$ for all irreducible Casselman-Wallach representations $\pi$ of $G$ and $\tau$ of $H$. This paper is concerned with the construction of explicit intertwining operators in $\operatorname{Hom}_H(\pi|_H,\tau)$, so-called symmetry breaking operators, in the case where both $\pi$ and $\tau$ are principal series representations. Such operators come in families that depend meromorphically on the induction parameters, and we show how to normalize them in order to make the parameter dependence holomorphic. This is done by establishing explicit Bernstein-Sato identities for their distribution kernels as well as explicit functional identities for the composition of symmetry breaking operators with standard Knapp-Stein intertwining operators for $G$ and $H$. We also show that the obtained normalization is optimal and identify a subset of parameters for which the family of operators vanishes. Finally, we relate the operators to the local archimedean Rankin-Selberg integrals and use this relation to evaluate them on the spherical vectors.

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arXiv2024

Realization of unitary representations of the Lorentz group on de Sitter space

This paper builds on our previous work in which we showed that, for all connected semisimple linear Lie groups $G$ acting on a non-compactly causal symmetric space $M = G/H$, every irreducible unitary representation of $G$ can be realized by boundary value maps of holomorphic extensions in distributional sections of a vector bundle over $M$. In the present paper we discuss this procedure for the connected Lorentz group $G = SO_{1,d}(R)_e$ acting on de Sitter space $M = dS^d$. We show in particular that the previously constructed nets of real subspaces satisfy the locality condition. Following ideas of Bros and Moschella from the 1990's, we show that the matrix-valued spherical function that corresponds to our extension process extends analytically to a large domain $G_C^{cut}$ in the complexified group $G_C = \SO_{1,d}(C)$, which for $d = 1$ specializes to the complex cut plane $C \setminus (-\infinity, 0]$. A number of special situations is discussed specifically: (a) The case $d = 1$, which closely corresponds to standard subspaces in Hilbert spaces, (b) the case of scalar-valued functions, which for $d > 2$ is the case of spherical representations, for which we also describe the jump singularities of the holomorphic extensions on the cut in de Sitter space, (c) the case $d = 3$, where we obtain rather explicit formulas for the matrix-valued spherical functions.

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arXiv2024

The holomorphic discrete series contribution to the generalized Whittaker Plancherel formula II. Non-tube type groups

For every simple Hermitian Lie group $G$, we consider a certain maximal parabolic subgroup whose unipotent radical $N$ is either abelian (if $G$ is of tube type) or two-step nilpotent (if $G$ is of non-tube type). By the generalized Whittaker Plancherel formula we mean the Plancherel decomposition of $L^2(G/N,\omega)$, the space of square-integrable sections of the homogeneous vector bundle over $G/N$ associated with an irreducible unitary representation $\omega$ of $N$. Assuming that the central character of $\omega$ is contained in a certain cone, we construct embeddings of all holomorphic discrete series representations of $G$ into $L^2(G/N,\omega)$ and show that the multiplicities are equal to the dimensions of the lowest $K$-types. The construction is in terms of a kernel function which can be explicitly defined using certain projections inside a complexification of $G$. This kernel function carries all information about the holomorphic discrete series embedding, the lowest $K$-type as functions on $G/N$, as well as the associated Whittaker vectors.

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arXiv2023

A Pairing Formula for Resonant States on Finite Regular Graphs

On a finite regular graph, (co)resonant states are eigendistributions of the transfer operator associated to the shift on one-sided infinite non-backtracking paths. We introduce two pairings of resonant and coresonant states, the vertex pairing which involves only the dependence on the initial/terminal vertex of the path, and the geodesic pairing which is given by integrating over all geodesics the evaluation of the coresonant state on the first half of the geodesic times the resonant state on the second half. The main result is that these two pairings coincide up to a constant which depends on the resonance, i.e. the corresponding eigenvalue of the transfer operator.

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arXiv2023

Edge Laplacians and Edge Poisson Transforms for Graphs

For a finite graph, we establish natural isomorphisms between eigenspaces of a Laplace operator acting on functions on the edges and eigenspaces of a transfer operator acting on functions on one-sided infinite non-backtracking paths. Interpreting the transfer operator as a classical dynamical system and the Laplace operator as its quantization, this result can be viewed as a quantum-classical correspondence. In contrast to previously established quantum-classical correspondences for the vertex Laplacian which exclude certain exceptional spectral parameters, our correspondence is valid for all parameters. This allows us to relate certain spectral quantities to topological properties of the graph such as the cyclomatic number and the 2-colorability. The quantum-classical correspondence for the edge Laplacian is induced by an edge Poisson transform on the universal covering of the graph which is a tree of bounded degree. In the special case of regular trees, we relate both the vertex and the edge Poisson transform to the representation theory of the automorphism group of the tree and study associated operator valued Hecke algebras.

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arXiv2023

Symmetry breaking for $\operatorname{PGL}(2)$ over non-archimedean local fields

For a quadratic extension $\mathbb{E}/\mathbb{F}$ of non-archimedean local fields we construct explicit holomorphic families of intertwining operators between principal series representations of $\operatorname{PGL}(2,\mathbb{E})$ and $\operatorname{PGL}(2,\mathbb{F})$, also referred to as symmetry breaking operators. These families are given in terms of their distribution kernels which can be viewed as distributions on $\mathbb{E}$ depending holomorphically on the principal series parameters. For all such parameters we determine the support of these distributions, and we study their mapping properties. This leads to a classification of all intertwining operators between principal series representations, not necessarily irreducible. As an application, we show that every Steinberg representation of $\operatorname{PGL}(2,\mathbb{E})$ contains a Steinberg representation of $\operatorname{PGL}(2,\mathbb{F})$ as a direct summand of Hilbert spaces.

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arXiv2023

Heisenberg parabolically induced representations of Hermitian Lie groups, Part II: Next-to-minimal representations and branching rules

Every simple Hermitian Lie group has a unique family of spherical representations induced from a maximal parabolic subgroup whose unipotent radical is a Heisenberg group. For most Hermitian groups, this family contains a complementary series, and at its endpoint sits a proper unitarizable subrepresentation. We show that this subrepresentation is next-to-minimal in the sense that its associated variety is a next-to-minimal nilpotent coadjoint orbit. Moreover, for the Hermitian groups $\operatorname{SO}_0(2,n)$ and $E_{6(-14)}$ we study some branching problems of these next-to-minimal representations.

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arXiv2023

Restricting holomorphic discrete series representations to a compact dual pair

The goal of this article is to study the branching problem for a holomorphic discrete series representation of the conformal group of a simple Euclidean Jordan algebra $V$ restricted to the subgroup $\operatorname{PSL}_2(\mathbb{R})\times\operatorname{Aut}(V)$ where $\operatorname{Aut}(V)$ denotes the compact group of automorphisms of $V$. We use a realization of the holomorphic discrete series on a space of vector-values $L^2$-functions as well as the stratified model developed by the second author to relate the branching problem to the decomposition of certain representations of the compact group $\operatorname{Aut}(V)$ and to vector-valued orthogonal polynomials.

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arXiv2023

Nets of standard subspaces on non-compactly causal symmetric spaces

Let G be a connected simple linear Lie group and H in G a symmetric subgroup such that the corresponding symmetric space G/H is non-compactly causal. We show that any irreducible unitary representation of G leads naturally to a net of standard subspaces on G/H that is isotone, covariant and has the Reeh--Schlieder and the Bisognano--Wichmann property. We also show that this result extends to the universal covering group of SL(2,R) which has some interesting application to intersections of standard subspaces associated to representations of such groups. For this a detailed study of hyperfunction and distribution vectors is needed. In particular we show that every H-finite hyperfunction vector is in fact a distribution vector.

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arXiv2023

Resonances and residue operators for pseudo-Riemannian hyperbolic spaces

For any pseudo-Riemannian hyperbolic space $X$ over $\mathbb{R},\mathbb{C},\mathbb{H}$ or $\mathbb{O}$, we show that the resolvent $R(z)=(\Box-z\operatorname{Id})^{-1}$ of the Laplace-Beltrami operator $-\Box$ on $X$ can be extended meromorphically across the spectrum of $\Box$ as a family of operators $C_c^\infty(X)\to \mathcal{D}'(X)$. Its poles are called resonances and we determine them explicitly in all cases. For each resonance, the image of the corresponding residue operator in $\mathcal{D}'(X)$ forms a representation of the isometry group of $X$, which we identify with a subrepresentation of a degenerate principal series. Our study includes in particular the case of even functions on de Sitter and Anti-de Sitter spaces. For Riemannian symmetric spaces analogous results were obtained by Miatello-Will and Hilgert-Pasquale. The main qualitative differences between the Riemannian and the non-Riemannian setting are that for non-Riemannian spaces the resolvent can have poles of order two, it can have a pole at the branching point of the covering to which $R(z)$ extends, and the residue representations can be infinite-dimensional.

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arXiv2022

Heisenberg parabolically induced representations of Hermitian Lie groups, Part I: Unitarity and subrepresentations

For a Hermitian Lie group $G$, we study the family of representations induced from a character of the maximal parabolic subgroup $P=MAN$ whose unipotent radical $N$ is a Heisenberg group. Realizing these representations in the non-compact picture on a space $I(\nu)$ of functions on the opposite unipotent radical $\bar{N}$, we apply the Heisenberg group Fourier transform mapping functions on $\bar N$ to operators on Fock spaces. The main result is an explicit expression for the Knapp-Stein intertwining operators $I(\nu)\to I(-\nu)$ on the Fourier transformed side. This gives a new construction of the complementary series and of certain unitarizable subrepresentations at points of reducibility. Further auxiliary results are a Bernstein-Sato identity for the Knapp-Stein kernel on $\bar{N}$ and the decomposition of the metaplectic representation under the non-compact group $M$.

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arXiv2022

Generalized Laguerre functions and Whittaker vectors for holomorphic discrete series

We study degenerate Whittaker vectors in scalar type holomorphic discrete series representations of tube type Hermitian Lie groups and their analytic continuation. In four different realizations, the bounded domain picture, the tube domain picture, the $L^2$-model and the Fock model, we find their explicit $K$-type expansions. The coefficients are expressed in terms of the generalized Laguerre functions on the corresponding symmetric cone, and we relate the $K$-type expansions to the formula for the generating function of the Laguerre polynomials and to their recurrence relations.

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arXiv2022

The holomorphic discrete series contribution to the generalized Whittaker Plancherel formula

For a Hermitian Lie group $G$ of tube type we find the contribution of the holomorphic discrete series to the Plancherel decomposition of the Whittaker space $L^2(G/N,\psi)$, where $N$ is the unipotent radical of the Siegel parabolic subgroup and $\psi$ is a certain non-degenerate unitary character on $N$. The holomorphic discrete series embeddings are constructed in terms of generalized Whittaker vectors for which we find explicit formulas in the bounded domain realization, the tube domain realization and the $L^2$-model of the holomorphic discrete series. Although $L^2(G/N,\psi)$ does not have finite multiplicities in general, the holomorphic discrete series contribution does. Moreover, we obtain an explicit formula for the formal dimensions of the holomorphic discrete series embeddings, and we interpret the holomorphic discrete series contribution to $L^2(G/N,\psi)$ as boundary values of holomorphic functions on a domain $\Xi$ in a complexification $G_{\mathbb{C}}$ of $G$ forming a Hardy type space $\mathcal{H}_2(\Xi,\psi)$.

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arXiv2021

The twisted Ruelle zeta function on compact hyperbolic orbisurfaces and Reidemeister-Turaev torsion

Let $X$ be a compact hyperbolic surface with finite order singularities, $X_1$ its unit tangent bundle. We consider the Ruelle zeta function $R(s;\rho)$ associated to a representation $\rho\colon\pi_1(X_1)\to\operatorname{GL}(V_\rho)$. If $\rho$ does not factor through $\pi_1(X)$, we show that the value at $0$ of the Ruelle zeta function equals the sign-refined Reidemeister-Turaev torsion of $(X_1, \rho)$ with respect to the Euler structure induced by the geodesic flow and to the natural homology orientation of $X_1$. It generalizes Fried's conjecture to non-unitary representations, and solves the phase and sign ambiguity in the unitary case. We also compute the vanishing order and the leading coefficient of the Ruelle zeta function at $s=0$ when $\rho$ factors through $\pi_1(X)$.

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