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## ‣ Sets of injectivity for weighted twisted spherical means and support theorems

Srivastava, R. K.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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In this article, we show that the spheres $S_R(o)=\{z\in\mathbb C^n: |z|=R\}$ are sets of injectivity for the weighted twisted spherical means (WTSM) for a suitable class of functions on $\mathbb C^n$. The weights here are spherical harmonics on $S^{2n-1}.$ In general, the question of set of injectivity for the twisted spherical means (TSM) with real analytic weight is still open. We would like to refer to \cite{NRR}, for some results on the sets of injectivity for the spherical means with real analytic weights in the Euclidean setup.; Comment: Published: J. Fourier Anal. Appl., 18(2012), no. 3, 592-608. arXiv admin note: text overlap with arXiv:1103.4571

## ‣ Note on a result of Kerman and Weit

Ray, Swagato K.; Sarkar, Rudra P.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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A result in \cite{Ker-Weit} states that a real valued continuous function $f$ on the circle and its nonnegative integral powers can generate a dense translation invariant subspace in the space of all continuous functions on the circle if $f$ has a unique maximum or a unique minimum. In this note we endeavour to show that this is quite a general phenomenon in harmonic analysis.; Comment: 5 pages

## ‣ A spectral gap property for subgroups of finite covolume in Lie groups

Bekka, Bachir; Cornulier, Yves
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Let G be a real Lie group and H a lattice or, more generally, a closed subgroup of finite covolume in G. We show that the unitary representation lambda_{G/H} of G on L^2(G/H) has a spectral gap, that is, the restriction of lambda_{G/H} to the orthogonal of the constants in L^2(G/H) does not have almost invariant vectors. This answers a question of G. Margulis. We give an application to the spectral geometry of locally symmetric Riemannian spaces of infinite volume.; Comment: 9 pages, no figure

## ‣ Nilpotent Gelfand pairs and spherical transforms of Schwartz functions III. Isomorphisms between Schwartz spaces under Vinberg's condition

Fischer, Veronique; Ricci, Fulvio; Yakimova, Oksana
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Let (N,K) be a nilpotent Gelfand pair, i.e., N is a nilpotent Lie group, K a compact group of automorphisms of N, and the algebra D(N)^K of left-invariant and K-invariant differential operators on N is commutative. In these hypotheses, N is necessarily of step at most two. We say that (N,K) satisfies Vinberg's condition if K acts irreducibly on $n/[n,n]$, where n= Lie(N). Fixing a system D of d formally self-adjoint generators of D(N)^K, the Gelfand spectrum of the commutative convolution algebra L^1(N)^K can be canonically identified with a closed subset S_D of R^d. We prove that, on a nilpotent Gelfand pair satisfying Vinberg's condition, the spherical transform establishes an isomorphism from the space of $K$-invariant Schwartz functions on N and the space of restrictions to S_D of Schwartz functions in R^d.; Comment: 51 pages

## ‣ The domain of the Fourier integral

Tibabishev, V. N.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We consider the problem of determining the Fourier integral in the Hilbert space of square integrable functions. Fourier integral is the scalar product of two functions belonging to the Hilbert space of square integrable functions and the Hilbert space of almost periodic functions. Scalar product for different Hilbert spaces defined at the intersection of these spaces, which contains only one zero element. Therefore, the Fourier integral is not defined in the Hilbert space of square integrable functions with nonzero norm.; Comment: PDF, 4 p

## ‣ Compact symmetric spaces, triangular factorization, and Cayley coordinates

Habermas, Derek
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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Let U/K represent a connected, compact symmetric space, where theta is an involution of U that fixes K, phi: U/K to U is the geodesic Cartan embedding, and G is the complexification of U. We investigate the intersection of phi(U/K) with the Bruhat decomposition of G corresponding to a theta-stable triangular, or LDU, factorization of the Lie algebra of G. When g in phi(U/K) is generic, the corresponding factorization g=ld(g)u is unique, where l in N^-, d(g) in H, and u in N^+. In this paper we present an explicit formula for d in Cayley coordinates, compute it in several types of symmetric spaces, and use it to identify representatives of the connected components of the generic part of phi(U/K). This formula calculates a moment map for a torus action on the highest dimensional symplectic leaves of the Evens-Lu Poisson structure on U/K.; Comment: 19 pages: Main proof entirely rewritten, sections reorganized, exposition made more precise and concise. To appear in Pacific Journal of Mathematics

## ‣ Complete monotonicity for inverse powers of some combinatorially defined polynomials

Scott, Alexander D.; Sokal, Alan D.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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We prove the complete monotonicity on $(0,\infty)^n$ for suitable inverse powers of the spanning-tree polynomials of graphs and, more generally, of the basis generating polynomials of certain classes of matroids. This generalizes a result of Szego and answers, among other things, a long-standing question of Lewy and Askey concerning the positivity of Taylor coefficients for certain rational functions. Our proofs are based on two_ab initio_ methods for proving that $P^{-\beta}$ is completely monotone on a convex cone $C$: the determinantal method and the quadratic-form method. These methods are closely connected with harmonic analysis on Euclidean Jordan algebras (or equivalently on symmetric cones). We furthermore have a variety of constructions that, given such polynomials, can create other ones with the same property: among these are algebraic analogues of the matroid operations of deletion, contraction, direct sum, parallel connection, series connection and 2-sum. The complete monotonicity of $P^{-\beta}$ for some $\beta > 0$ can be viewed as a strong quantitative version of the half-plane property (Hurwitz stability) for $P$, and is also related to the Rayleigh property for matroids.; Comment: LaTeX2e, 70 pages (v2) or 82 pages (v3). Version 2 (accepted for publication in Acta Mathematica) is significantly reorganized at the suggestion of a referee; also...

## ‣ Quantitative property A, Poincare inequalities, L^p-compression and L^p-distortion for metric measure spaces

Tessera, Romain
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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We introduce a quantitative version of Property A in order to estimate the L^p-compressions of a metric measure space X. We obtain various estimates for spaces with sub-exponential volume growth. This quantitative property A also appears to be useful to yield upper bounds on the L^p-distortion of finite metric spaces. Namely, we obtain new optimal results for finite subsets of homogeneous Riemannian manifolds. We also introduce a general form of Poincare inequalities that provide constraints on compressions, and lower bounds on distortion. These inequalities are used to prove the optimality of some of our results.; Comment: 26 pages

## ‣ When is a Riesz distribution a complex measure?

Sokal, Alan D.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
Let R_\alpha be the Riesz distribution on a simple Euclidean Jordan algebra, parametrized by the complex number \alpha. I give an elementary proof of the necessary and sufficient condition for R_\alpha to be a locally finite complex measure (= complex Radon measure).; Comment: LaTeX2e, 15 pages. Version 2 contains some small changes suggested by a referee

## ‣ Ramanujan's Master Theorem for Riemannian symmetric spaces

Olafsson, Gestur; Pasquale, Angela
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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Ramanujan's Master theorem states that, under suitable conditions, the Mellin transform of a power series provides an interpolation formula for the coefficients of this series. Based on the duality of Riemannian symmetric spaces of compact and noncompact type inside a common complexification, we prove an analogue of Ramanujan's Master Theorem for the spherical Fourier transform of a spherical Fourier series. This extend the results proven by Bertram for Riemannian symmetric spaces of rank-one.; Comment: As accepted by JFA

## ‣ Invariant differential operators on nonreductive homogeneous spaces

Koornwinder, Tom H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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A systematic exposition is given of the theory of invariant differential operators on a not necessarily reductive homogeneous space. This exposition is modelled on Helgason's treatment of the general reductive case and the special non-reductive case of the space of horocycles. As a final application the differential operators on (not a priori reductive) isotropic pseudo-Riemannian spaces are characterized.; Comment: 11 pages, electronic version of old 1981 report

## ‣ Wavelets in Banach Spaces

Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
We describe a construction of wavelets (coherent states) in Banach spaces generated by admissible'' group representations. Our main targets are applications in pure mathematics while connections with quantum mechanics are mentioned. As an example we consider operator valued Segal-Bargmann type spaces and the Weyl functional calculus. Keywords: Wavelets, coherent states, Banach spaces, group representations, covariant, contravariant (Wick) symbols, Heisenberg group, Segal-Bargmann spaces, Weyl functional calculus (quantization), second quantization, bosonic field.; Comment: 37 pages; LaTeX2e; no pictures; 27/07/99: many small corrections

## ‣ Fourier restriction Theorem and characterization of weak $L^2$ eigenfunctions of the Laplace--Beltrami operator

Kumar, Pratyoosh
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
27.03434%
In this paper we prove the Fourier restriction theorem for $p=2$ on Riemannian symmetric spaces of noncompact type with real rank one which extends the earlier result proved in \cite[Theorem 1.1]{KRS}. This result depends on the weak $L^2$ estimates of the Poisson transform of $L^2$ function. By using this estimate of the Poisson transform we also characterizes all weak $L^2$ eigenfunction of the Laplace--Beltrami operator of Riemannian symmetric spaces of noncompact type with real rank one and eigenvalue $-(\lambda^2+\rho^2)$ for $\lambda\in\R\setminus\{0\}$.

## ‣ Crested products of Markov chains

D'Angeli, Daniele; Donno, Alfredo
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
27.03434%
In this work we define two kinds of crested product for reversible Markov chains, which naturally appear as a generalization of the case of crossed and nested product, as in association schemes theory, even if we do a construction that seems to be more general and simple. Although the crossed and nested product are inspired by the study of Gelfand pairs associated with the direct and the wreath product of two groups, the crested products are a more general construction, independent from the Gelfand pairs theory, for which a complete spectral theory is developed. Moreover, the $k$-step transition probability is given. It is remarkable that these Markov chains describe some classical models (Ehrenfest diffusion model, Bernoulli--Laplace diffusion model, exclusion model) and give some generalization of them. As a particular case of nested product, one gets the classical Insect Markov chain on the ultrametric space. Finally, in the context of the second crested product, we present a generalization of this Markov chain to the case of many insects and give the corresponding spectral decomposition.; Comment: Published in at http://dx.doi.org/10.1214/08-AAP546 the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org)

## ‣ On the Uniqueness of Solutions of the Schr\"odinger Equation on Riemannian Symmetric Spaces of the Noncompact Type

Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
Let X be a Riemannian symmetric space of the noncompact type. We prove that the solution of the time-dependent Schr\"odinger equation on X with square integrable initial condition f is identically zero at all times t whenever f and the solution at a time t0 > 0 are simultaneously very rapidly decreasing. The stated condition of rapid decrease is of Beurling type. Conditions respectively of Gelfand-Shilov, Cowling-Price and Hardy type are deduced.; Comment: 20 pages, To appear in Annales de l Institut Fourier

## ‣ A Lie-algebraic approach to the local index theorem on compact homogeneous spaces

Hong, Seunghun
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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Using a K-theory point of view, Bott related the Atiyah-Singer index theorem for elliptic operators on compact homogeneous spaces to the Weyl character formula. This article explains how to prove the local index theorem for compact homogenous spaces using Lie algebra methods. The method follows in outline the proof of the local index theorem due to Berline and Vergne. But the use of Kostant's cubic Dirac operator in place of the Riemannian Dirac operator leads to substantial simplifications. An important role is also played by the quantum Weil algebra of Alekseev and Meinrenken.; Comment: reduced in length; error in Prop. 5.4 [v2] corrected (now Prop. 4.4)

## ‣ Coxeter system of lines are sets of injectivity for the twisted spherical means on $\mathbb C$

Srivastava, R. K.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
It is well known that a line in $\mathbb R^2$ is not a set of injectivity for the spherical means for odd functions about that line. We prove that any line passing through the origin is a set of injectivity for the twisted spherical means (TSM) for functions $f\in L^2(\mathbb C),$ whose each of spectral projection $e^{\frac{1}{4}|z|^2}f\times\varphi_k$ is a polynomial. Then, we prove that any Coxeter system of even number of lines is a set of injectivity for the TSM for $L^q(\mathbb C),~1\leq q\leq2.$; Comment: This article is accepted in J. Funct. Anal. jointly with arXiv:1204.2773

## ‣ Two body problem on two point homogeneous spaces, invariant differential operators and the mass center concept

Shchepetilov, Alexey V.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
We consider the two body problem with central interaction on two point homogeneous spaces from point of view of the invariant differential operators theory. The representation of the two particle Hamiltonian in terms of the radial differential operator and invariant operators on the symmetry group is found. The connection of different mass center definitions for these spaces to the obtained expression for Hamiltonian operator is studied.; Comment: 26 pages, LaTeX, no figures, text improved

## ‣ Spectral multipliers for the Kohn Laplacian on forms on the sphere in $\mathbb{C}^n$

Casarino, Valentina; Cowling, Michael G.; Martini, Alessio; Sikora, Adam
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
The unit sphere $\mathbb{S}$ in $\mathbb{C}^n$ is equipped with the tangential Cauchy-Riemann complex and the associated Laplacian $\Box_b$. We prove a H\"ormander spectral multiplier theorem for $\Box_b$ with critical index $n-1/2$, that is, half the topological dimension of $\mathbb{S}$. Our proof is mainly based on representation theory and on a detailed analysis of the spaces of differential forms on $\mathbb{S}$.; Comment: 28 pages

## ‣ Eigenmodes of Lens and Prism Spaces

Lehoucq, Roland; Uzan, Jean-Philippe; Weeks, Jeffrey
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.03434%
Cosmologists are taking a renewed interest in multiconnected spherical 3-manifolds (spherical spaceforms) as possible models for the physical universe. To understand the formation of large scale structures in such a universe, cosmologists express physical quantities, such as density fluctuations in the primordial plasma, as linear combinations of the eigenmodes of the Laplacian, which can then be integrated forward in time. This need for explicit eigenmodes contrasts sharply with previous mathematical investigations, which have focused on questions of isospectrality rather than eigenmodes. The present article provides explicit orthonormal bases for the eigenmodes of lens and prism spaces. As a corollary it extends known results on spectra from homogeneous lens spaces L(p,1) [Ikeda 1995] to arbitrary lens spaces L(p,q).; Comment: 19 pages, 3 figures