arXiv subjects

Joseph A. Wolf

Publications and source records attributed to Joseph A. Wolf.

At least 37 records · Page 2Linked to original sources

arXiv2016

Cycle Spaces of Infinite Dimensional Flag Domains

Let $G$ be a complex simple direct limit group, specifically $SL(\infty;\mathbb{C})$, $SO(\infty;\mathbb{C})$ or $Sp(\infty;\mathbb{C})$. Let $\mathcal{F}$ be a (generalized) flag in $\mathbb{C}^\infty$. If $G$ is $SO(\infty;\mathbb{C})$ or $Sp(\infty;\mathbb{C})$ we suppose further that $\mathcal{F}$ is isotropic. Let $\mathcal{Z}$ denote the corresponding flag manifold; thus $\mathcal{Z} = G/Q$ where $Q$ is a parabolic subgroup of $G$. In a recent paper with Ignatyev and Penkov, we studied real forms $G_0$ of $G$ and properties of their orbits on $\mathcal{Z}$. Here we concentrate on open $G_0$--orbits $D \subset \mathcal{Z}$. When $G_0$ is of hermitian type we work out the complete $G_0$--orbit structure of flag manifolds dual to the bounded symmetric domain for $G_0$. Then we develop the structure of the corresponding cycle spaces $\mathcal{M}_D$. Finally we study the real and quaternionic analogs of these theories. All this extends an large body of results from the finite dimensional cases on the structure of hermitian symmetric spaces and related cycle spaces.

math.DG↗

arXiv2016

Killing Vector Fields of Constant Length on Riemannian Normal Homogeneous Spaces

Killing vector fields of constant length correspond to isometries of constant displacement. Those in turn have been used to study homogeneity of Riemannian and Finsler quotient manifolds. Almost all of that work has been done for group manifolds or, more generally, for symmetric spaces. This paper extends the scope of research on constant length Killing vector fields to a class of Riemannian normal homogeneous spaces.

math.DG↗

arXiv2015

Stepwise Square Integrable Representations: the Concept and Some Consequences

There are some new developments on Plancherel formula and growth of matrix coefficients for unitary representations of nilpotent Lie groups. These have several consequences for the geometry of weakly symmetric spaces and analysis on parabolic subgroups of real semisimple Lie groups, and to (infinite dimensional) locally nilpotent Lie groups. Many of these consequences are still under development. In this note I'll survey a few of these new aspects of representation theory for nilpotent Lie groups and parabolic subgroups.

math.RT↗

arXiv2015

Bounded Isometries and Homogeneous Quotients

In this paper we give an explicit description of the bounded displacement isometries of a class of spaces that includes the Riemannian nilmanifolds. The class of spaces consists of metric spaces (and thus includes Finsler manifolds) on which an exponential solvable Lie group acts transitively by isometries. The bounded isometries are proved to be of constant displacement. Their characterization gives further evidence for the author's 1962 conjecture on homogeneous Riemannian quotient manifolds. That conjecture suggests that if $Γ\backslash M$ is a Riemannian quotient of a connected simply connected homogeneous Riemannian manifold $M$, then $Γ\backslash M$ is homogeneous if and only if each isometry $γ\in Γ$ is of constant displacement. Our description of bounded isometries gives an alternative proof of an old result of J. Tits on bounded automorphisms of semisimple Lie groups. The topic of constant displacement isometries has an interesting history, starting with Clifford's use of quaternions in non--euclidean geometry, and we sketch that in a historical note.

math.DG↗

arXiv2015

Toward a Classification of Killing Vector Fields of Constant Length on Pseudo--Riemannian Normal Homogeneous Spaces

In this paper we develop the basic tools for a classification of Killing vector fields of constant length on pseudo--riemannian homogeneous spaces. This extends a recent paper of M. Xu and J. A. Wolf, which classified the pairs $(M,ξ)$ where $M = G/H$ is a Riemannian normal homogeneous space, $G$ is a compact simple Lie group, and $ξ\in \mathfrak{g}$ defines a nonzero Killing vector field of constant length on $M$. The method there was direct computation. Here we make use of the moment map $M \to \mathfrak{g}^*$ and the flag manifold structure of Ad(G)$ξ$ to give a shorter, more geometric proof which does not require compactness and which is valid in the pseudo--riemannian setting. In that context we break the classification problem into three parts. The first is easily settled. The second concerns the cases where $ξ$ is elliptic and $G$ is simple (but not necessarily compact); that case is our main result here. The third, which remains open, is a more combinatorial problem involving elements of the first two.

math.DG↗

arXiv2015

Sp(2)/U(1) and a Positive Curvature Problem

A compact Riemannian homogeneous space $G/H$, with a bi--invariant orthogonal decomposition $\mathfrak{g}=\mathfrak{h}+\mathfrak{m}$ is called positively curved for commuting pairs, if the sectional curvature vanishes for any tangent plane in $T_{eH}(G/H)$ spanned by a linearly independent commuting pair in $\mathfrak{m}$. In this paper,we will prove that on the coset space $\mathrm{Sp}(2)/\mathrm{U}(1)$, in which $\mathrm{U}(1)$ corresponds to a short root, admits positively curved metrics for commuting pairs. B. Wilking recently proved that this $\mathrm{Sp}(2)/\mathrm{U}(1)$ can not be positively curved in the general sense. This is the first example to distinguish the set of compact coset spaces admitting positively curved metrics, and that for metrics positively curved only for commuting pairs.

math.DG↗

arXiv2014

On the Analytic Structure of Commutative Nilmanifolds

In the classification theorems of Vinberg and Yakimova for commutative nilmanifolds, the relevant nilpotent groups have a very surprising analytic property. The manifolds are of the form $G/K = N \rtimes K/K$ where, in all but three cases, the nilpotent group $N$ has irreducible unitary representations whose coefficients are square integrable modulo the center $Z$ of $N$. Here we show that, in those three "exceptional" cases, the group $N$ is a semidirect product $N_1 \rtimes \mathbb{R}$ or $N_1 \rtimes \mathbb{C}$ where the normal subgroup $N_1$ contains the center $Z$ of $N$ and has irreducible unitary representations whose coefficients are square integrable modulo $Z$. This leads directly to explicit harmonic analysis and Fourier inversion formulae for commutative nilmanifolds.

math.RT↗

arXiv2014

Stepwise Square Integrable Representations for Locally Nilpotent Lie Groups

In a recent paper we found conditions for a nilpotent Lie group $N$ to have a filtration by normal subgroups whose successive quotients have square integrable representations, and such that these square integrable representations fit together nicely to give an explicit construction of Plancherel almost all representations of $N$. That resulted in explicit character formulae, Plancherel formulae and multiplicity formulae. We also showed that nilradicals $N$ of minimal parabolic subgroups $P = MAN$ enjoy that "stepwise square integrable" property. Here we extend those results to direct limits of stepwise square integrable nilpotent Lie groups. This involves some development of the corresponding Schwartz spaces. The main result is an explicit Fourier inversion formula for that class of infinite dimensional Lie groups. One important consequence is the Fourier inversion formula for nilradicals of classical minimal parabolic subgroups of finitary real reductive Lie groups such as $GL(\infty;R)$, $Sp(\infty;C)$ and $SO(\infty,\infty)$.

math.RT↗

arXiv2013

The Plancherel Formula for Minimal Parabolic Subgroups

In a recent paper we found conditions for a nilpotent Lie group to be foliated into subgroups that have square integrable unitary representations that fit together to form a filtration by normal subgroups. That resulted in explicit character formulae, Plancherel formulae and multiplicity formulae. We also showed that nilradicals $N$ of minimal parabolic subgroups $P = MAN$ enjoy that "stepwise square integrable" property. Here we extend those results from $N$ to $P$. The Pfaffian polynomials, which give orthogonality relations and Plancherel density for $N$, also give a semiinvariant differential operator that compensates lack of unimodularity for $P$. The result is a completely explicit Plancherel formula for $P$.

math.RT↗

arXiv2012

Direct Systems of Spherical Functions and Representations

Spherical representations and functions are the building blocks for harmonic analysis on riemannian symmetric spaces. In this paper we consider spherical functions and spherical representations related to certain infinite dimensional symmetric spaces $G_\infty/K_\infty = \varinjlim G_n/K_n$. We use the representation theoretic construction $ϕ(x) = $ where $e$ is a $K_\infty$--fixed unit vector for $π$. Specifically, we look at representations $π_\infty = \varinjlim π_n$ of $G_\infty$ where $π_n$ is $K_n$--spherical, so the spherical representations $π_n$ and the corresponding spherical functions $ϕ_n$ are related by $ϕ_n(x) = $ where $e_n$ is a $K_n$--fixed unit vector for $π_n$, and we consider the possibility of constructing a $K_\infty$--spherical function $ϕ_\infty = \lim ϕ_n$. We settle that matter by proving the equivalence of condtions (i) $\{e_n\}$ converges to a nonzero $K_\infty$--fixed vector $e$, and (ii) $G_\infty/K_\infty$ has finite symmetric space rank (equivalently, it is the Grassmann manifold of $p$--planes in $\F^\infty$ where $p < \infty$ and $\F$ is $\R$, $\C$ or $\H$). In that finite rank case we also prove the functional equation $ϕ(x)ϕ(y) = \lim_{n\to \infty} \int_{K_n}ϕ(xky)dk$ of Faraut and Olshanskii, which is their definition of spherical functions.

math.RT↗

arXiv2012

Principal series representations of infinite dimensional Lie groups, II: Construction of induced representations

We study representations of the classical infinite dimensional real simple Lie groups $G$ induced from factor representations of minimal parabolic subgroups $P$. This makes strong use of the recently developed structure theory for those parabolic subgroups and subalgebras. In general parabolics in the infinite dimensional classical Lie groups are are somewhat more complicated than in the finite dimensional case, and are not direct limits of finite dimensional parabolics. We extend their structure theory and use it for the infinite dimensional analog of the classical principal series representations. In order to do this we examine two types of conditions on $P$: the flag-closed condition and minimality. We use some riemannian symmetric space theory to prove that if $P$ is flag-closed then any maximal lim-compact subgroup $K$ of $G$ is transitive on $G /P$\,. When $P$ is minimal we prove that it is amenable, and we use properties of amenable groups to induce unitary representations $τ$ of $P$ up to continuous representations $\Ind_P^G(τ)$ of $G$ on complete locally convex topological vector spaces. When $P$ is both minimal and flag-closed we have a decomposition $P = MAN$ similar to that of the finite dimensional case, and we show how this gives $K$--spectrum information $\Ind_P^G(τ)|_K = \Ind_M^K(τ|_M)$.

math.RT↗

arXiv2012

Separating Vector Bundle Sections by Invariant Means

We sharpen the construction of representation space in the paper "Principal Series Representations of Infinite Dimensional Lie Groups II: Construction of Induced Representations". We show that the principal series representation spaces constructed there, are completions of spaces of sections of Hilbert bundles rather than completions of quotient spaces of sections.

math.RT↗

arXiv2012

Locally symmetric homogeneous Finsler spaces

Let $(M,F)$ be a connected Finsler space and $d$ the distance function of $(M,F)$. A Clifford translation is an isometry $ρ$ of $(M,F)$ of constant displacement, in other words such that $d(x,ρ(x))$ is a constant function on $M$. In this paper we consider a connected simply connected symmetric Finsler space and a discrete subgroup $Γ$ of the full group of isometries. We prove that the quotient manifold $(M, F)/Γ$ is a homogeneous Finsler space if and only if $Γ$ consists of Clifford translations of $(M,F)$. In the process of the proof of the main theorem, we classify all the Clifford translations of symmetric Finsler spaces.

math.DG↗

arXiv2012

Principal Series Representations of Infinite Dimensional Lie Groups, I: Minimal Parabolic Subgroups

We study the structure of minimal parabolic subgroups of the classical infinite dimensional real simple Lie groups, corresponding to the classical simple direct limit Lie algebras. This depends on the recently developed structure of parabolic subgroups and subalgebras that are not necessarily direct limits of finite dimensional parabolics. We then discuss the use of that structure theory for the infinite dimensional analog of the classical principal series representations. We look at the unitary representation theory of the classical lim--compact groups $U(\infty)$, $SO(\infty)$ and $Sp(\infty)$ in order to construct the inducing representations, and we indicate some of the analytic considerations in the actual construction of the induced representations.

math.RT↗

arXiv2012

A duality for the double fibration transform

We establish a duality within the spectral sequence that governs the holomorphic double fibration transform. It has immediate application to the questions of injectivity and range characterization for this transform. We discuss some key examples and an improved duality that holds in the Hermitian holomorphic case.

math.RT↗

arXiv2011

Pseudo-Riemannian Weakly Symmetric Manifolds

There is a well developed theory of weakly symmetric Riemannian manifolds. Here it is shown that several results in the Riemannian case are also valid for weakly symmetric pseudo-Riemannian manifolds, but some require additional hypotheses. The topics discussed are homogeneity, geodesic completeness, the geodesic orbit property, weak symmetries, and the structure of the nilradical of the isometry group. Also, we give a number of examples of weakly symmetric pseudo-Riemannian manifolds, some mirroring the Riemannian case and some indicating the problems in extending Riemannian results to weakly symmetric pseudo-Riemannian spaces.

math.DG↗

arXiv2011

The Paley-Wiener Theorem and Limits of Symmetric Spaces

We extend the Paley--Wiener theorem for riemannian symmetric spaces to an important class of infinite dimensional symmetric spaces. For this we define a notion of propagation of symmetric spaces and examine the direct (injective) limit symmetric spaces defined by propagation. This relies on some of our earlier work on invariant differential operators and the action of Weyl group invariant polynomials under restriction.

math.RT↗

arXiv2010

Classical Analysis and Nilpotent Lie Groups

Classical Fourier analysis has an exact counterpart in group theory and in some areas of geometry. Here I'll describe how this goes for nilpotent Lie groups and for a class of Riemannian manifolds closely related to a nilpotent Lie group structure. There are also some infinite dimensional analogs but I won't go into that here. The analytic ideas are not so different from those of the classical Fourier transform and Fourier inversion theories in one real variable.

math.CA↗