arXiv subjects

Pablo Ramacher

Publications and source records attributed to Pablo Ramacher.

27 records · Page 2Linked to original sources

arXiv2011

Integral operators on the Oshima compactification of a Riemannian symmetric space of non-compact type. Microlocal analysis and kernel asymptotics

Let $\X\simeq G/K$ be a Riemannian symmetric space of non-compact type, $\widetilde \X$ its Oshima compactification, and $(π,\mathrm{C}(\widetilde \X))$ the regular representation of $G$ on $\widetilde \X$. We study integral operators on $\widetilde \X$ of the form $π(f)$, where $f$ is a rapidly falling function on $G$, and characterize them within the framework of pseudodifferential operators, describing the singular nature of their kernels. In particular, we consider the holomorphic semigroup generated by a strongly elliptic operator associated to the representation $π$, as well as its resolvent, and describe the asymptotic behavior of the corresponding semigroup and resolvent kernels.

math.DG↗

arXiv2009

Singular equivariant asymptotics and the moment map II

This is the second of a series of papers dealing with the asymptotic behavior of certain integrals occuring in the description of the spectrum of an invariant elliptic operator on a compact Riemannian manifold carrying the action of a compact, connected Lie group of isometries, and in the study of its equivariant cohomology via the moment map.

math.SG↗

arXiv2009

Singular equivariant asymptotics and the moment map I

This is the first of a series of papers dealing with the asymptotic behavior of certain integrals occuring in the description of the spectrum of an invariant elliptic operator on a compact Riemannian manifold carrying the action of a compact, connected Lie group of isometries, and in the study of its equivariant cohomology via the moment map.

math.SG↗

arXiv2007

Reduced Weyl asymptotics for pseudodifferential operators on bounded domains II. The compact group case

Let $G\subset Ø(n)$ be a compact group of isometries acting on $n$-dimensional Euclidean space $\R^n$, and ${\bf{X}}$ a bounded domain in $\R^n$ which is transformed into itself under the action of $G$. Consider a symmetric, classical pseudodifferential operator $A_0$ in $Ł^2(\R^n)$ that commutes with the regular representation of $G$, and assume that it is elliptic on $\bf{X}$. We show that the spectrum of the Friedrichs extension $A$ of the operator $\mathrm{res} \circ A_0 \circ \mathrm{ext}: \CT({\bf{X}}) \to Ł^2({\bf{X}})$ is discrete, and using the method of the stationary phase, we derive asymptotics for the number $N_χ(λ)$ of eigenvalues of $A$ equal or less than $λ$ and with eigenfunctions in the $χ$-isotypic component of $Ł^2({\bf{X}})$ as $λ\to \infty$, giving also an estimate for the remainder term for singular group actions. Since the considered critical set is a singular variety, we recur to partial desingularization in order to apply the stationary phase theorem.

math.AP↗

arXiv2007

Reduced Weyl asymptotics for pseudodifferential operators on bounded domains I. The finite group case

Let $G\subset Ø(n)$ be a group of isometries acting on $n$-dimensional Euclidean space $\R^n$, and ${\bf{X}}$ a bounded domain in $\R^n$ which is transformed into itself under the action of G. Consider a symmetric, classical pseudodifferential operator A_0 in $Ł^2(\R^n)$ with G-invariant Weyl symbol, and assume that it is semi-bounded from below. We show that the spectrum of the Friedrichs extension A of the operator $\mathrm{res} \circ A_0 \circ \mathrm{ext}: \CT({\bf{X}}) \to Ł^2({\bf{X}})$ is discrete, and derive asymptotics for the number $N_χ(λ)$ of eigenvalues of A less or equal $λ$ and with eigenfunctions in the $χ$-isotypic component of $Ł^2({\bf{X}})$, giving also an estimate for the remainder term in both cases where G is a finite, or, more generally, a compact group. In particular, we show that the multiplicity of each unitary irreducible representation in $Ł^2({\bf{X}})$ is asymptotically proportional to its dimension.

math.AP↗

arXiv2004

Pseudodifferential operators on prehomogeneous vector spaces

Let $G_\C$ be a connected, linear algebraic group defined over $\R$, acting regularly on a finite dimensional vector space $V_\C$ over $\C$ with $\R$-structure $V_\R$. Assume that $V_\C$ posseses a Zariski-dense orbit, so that $(G_\C,ρ,V_\C)$ becomes a prehomogeneous vector space over $\R$. We consider the left regular representation $π$ of the group of $\R$-rational points $G_\R$ on the Banach space $\Cvan(V_\R)$ of continuous functions on $V_\R$ vanishing at infinity, and study the convolution operators $π(f)$, where $f$ is a rapidly decreasing function on the identity component of $G_\R$. Denote the complement of the dense orbit by $S_\C$, and put $S_\R=S_\C\cap V_\R$. It turns out that the restriction of $π(f)$ to $V_\R-S_\R$ is a smooth operator. Furthermore, if $G_\C$ is reductive, and $S_\C$ and $S_\R$ are irreducible hypersurfaces, $π(f)$ corresponds, on each connected component of $V_\R-S_\R$, to a totally characteristic pseudodifferential operator. We then investigate the restriction of the Schwartz kernel of $π(f)$ to the diagonal. It defines a distribution on $V_\R-S_\R$ given by some power $|p(m)|^s$ of a relative invariant $p(m)$ of $(G_\C,ρ,V_\C)$ and, as a consequence of the fundamental theorem of prehomogeneous vector spaces, its extension to $V_\R$, and the complex $s$-plane, satisfies functional equations. A trace of $π(f)$ can then be defined by subtracting the singular contributions of the poles of the meromorphic extension.

math.RT↗

arXiv2003

Analysis on real affine G-varieties

We consider the action of a real linear algebraic group $G$ on a smooth, real affine algebraic variety $M\subset \R^n$, and study the corresponding left regular $G$-representation on the Banach space $C_0(M)$ of continuous, complex valued functions on $M$ vanishing at infinity. We show that the differential structure of this representation is already completely characterized by the action of the Lie algebra $\g$ of $G$ on the dense subspace $¶=\C[M] \cdot e^{-r^2}$, where $\C[M]$ denotes the algebra of regular functions of $M$ and $r$ the distance function in $\R^n$. We prove that the elements of this subspace constitute analytic vectors of the considered $G$-representation, and, using this fact, we construct discrete reducing series in $C_0(M)$. In case that $G$ is reductive, $K$ a maximal compact subgroup, $¶$ turns out to be a $(\g,K)$-module in the sense of Harish-Chandra and Lepowsky, and by taking suitable subquotients of $¶$, respectively $C_0(M)$, one gets admissible $(\g,K)$-modules as well as $K$-finite Banach representations.

math.RT↗

arXiv2001

Geometric and analytic properties of families of hypersurfaces in Eguchi-Hanson space

We study the geometry of families of hypersurfaces in Eguchi-Hanson space that arise as complex line bundles over curves in $S^2$ and are three-dimensional, non-compact Riemannian manifolds, which are foliated in Hopf tori for closed curves. They are negatively curved, asymptotically flat spaces, and we compute the complete three-dimensional curvature tensor as well as the second fundamental form, giving also some results concerning their geodesic flow. We show the non-existence of $Ł^p$-harmonic functions on these hypersurfaces for every $p \geq 1$ and arbitrary curves, and determine the infima of the essential spectra of the Laplace and of the square of the Dirac operator in the case of closed curves. For circles we also compute the $Ł^2$-kernel of the Dirac operator in the sense of spectral theory and show that it is infinite dimensional. We consider further the Einstein Dirac system on these spaces and construct explicit examples of T-Killing spinors on them.

math.DG↗

arXiv1999

Localization of elementary systems in the theory of Wigner

Starting from Wigner's theory of elementary systems and following a recent approach of Schroer we define certain subspaces of localized wave functions in the underlying Hilbert space with the help of the theory of modular von-Neumann algebras of Tomita and Takesaki. We characterize the elements of these subspaces as boundary values of holomorphic functions in the sense of distribution theory and show that the corresponding holomorphic functions satisfy the sufficient conditions of the theorems of Paley-Wiener-Schwartz and Hörmander.

math.RT↗