Results for "Olaf Post"

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On a generalisation of Krein's exampleOct 30 2017We generalise a classical example given by Krein in 1953. We compute the difference of the resolvents and the difference of the spectral projections explicitly. We further give a full description of the unitary invariants, i.e., of the spectrum and the ... More
First order approach and index theorems for discrete and metric graphsAug 28 2007Sep 03 2007The aim of the present paper is to introduce the notion of first order (supersymmetric) Dirac operators on discrete and metric (``quantum'') graphs. In order to cover all self-adjoint boundary conditions for the associated metric graph Laplacian, we develop ... More
Branched quantum wave guides with Dirichlet boundary conditions: the decoupling caseFeb 03 2005We consider a family of open sets $M_\epsilon$ which shrinks with respect to an appropriate parameter $\epsilon$ to a graph. Under the additional assumption that the vertex neighbourhoods are small we show that the appropriately shifted Dirichlet spectrum ... More
Abstract graph-like space and vector-valued metric graphsMar 30 2016In this note we present some abstract ideas how one can construct spaces from building blocks according to a graph. The coupling is expressed via boundary pairs, and can be applied to very different spaces such as discrete graphs, quantum graphs or graph-like ... More
Spectral analysis of metric graphs and related spacesDec 10 2007Feb 15 2008The aim of the present article is to give an overview of spectral theory on metric graphs guided by spectral geometry on discrete graphs and manifolds. We present the basic concept of metric graphs and natural Laplacians acting on it and explicitly allow ... More
Equilateral quantum graphs and boundary triplesDec 10 2007Jan 15 2008The aim of the present paper is to analyse the spectrum of Laplace and Dirac type operators on metric graphs. In particular, we show for equilateral graphs how the spectrum (up to exceptional eigenvalues) can be described by a natural generalisation of ... More
Spectral convergence of non-compact quasi-one-dimensional spacesDec 22 2005Jan 02 2006We consider a family of non-compact manifolds $X_\eps$ (``graph-like manifolds'') approaching a metric graph $X_0$ and establish convergence results of the related natural operators, namely the (Neumann) Laplacian $\laplacian {X_\eps}$ and the generalised ... More
Periodic Manifolds with Spectral GapsJul 14 2002We investigate spectral properties of the Laplace operator on a class of non-compact Riemannian manifolds. For a given number $N$ we construct periodic (i.e. covering) manifolds such that the essential spectrum of the corresponding Laplacian has at least ... More
First order operators and boundary triplesNov 14 2007Dec 06 2007The aim of the present paper is to introduce a first order approach to the abstract concept of boundary triples for Laplace operators. Our main application is the Laplace operator on a manifold with boundary; a case in which the ordinary concept of boundary ... More
Spectral gaps and discrete magnetic LaplaciansOct 03 2017Aug 07 2018The aim of this article is to give a simple geometric condition that guarantees the existence of spectral gaps of the discrete Laplacian on periodic graphs. For proving this, we analyse the discrete magnetic Laplacian (DML) on the finite quotient and ... More
Boundary pairs associated with quadratic formsOct 17 2012May 04 2015We introduce an abstract framework for elliptic boundary value problems in a variational form. Given a non-negative quadratic form in a Hilbert space, a boundary pair consists of a bounded operator, the boundary operator, and an auxiliary Hilbert space, ... More
Eigenvalues in Spectral Gaps of a Perturbed Periodic ManifoldJul 14 2002We consider a non-compact Riemannian periodic manifold such that the corresponding Laplacian has a spectral gap. By continuously perturbing the periodic metric locally we can prove the existence of eigenvalues in a gap. A lower bound on the number of ... More
Convergence of resonances on thin branched quantum wave guidesFeb 21 2007We prove an abstract criterion stating resolvent convergence in the case of operators acting in different Hilbert spaces. This result is then applied to the case of Laplacians on a family $X_\eps$ of branched quantum waveguides. Combining it with an exterior ... More
Quantum networks modelled by graphsJun 04 2007Dec 10 2007Quantum networks are often modelled using Schroedinger operators on metric graphs. To give meaning to such models one has to know how to interpret the boundary conditions which match the wave functions at the graph vertices. In this article we give a ... More
Convergence of spectra of graph-like thin manifoldsDec 10 2003Aug 16 2004We consider a family of compact manifolds which shrinks with respect to an appropriate parameter to a graph. The main result is that the spectrum of the Laplace-Beltrami operator converges to the spectrum of the (differential) Laplacian on the graph with ... More
Approximation of fractals by manifolds and other graph-like spacesFeb 08 2018We define a distance between energy forms on a graph-like metric measure space and on a discrete weighted graph using the concept of quasi-unitary equivalence. We apply this result to metric graphs and graph-like manifolds (e.g. a small neighbourhood ... More
A general approximation of quantum graph vertex couplings by scaled Schroedinger operators on thin branched manifoldsMay 23 2012Sep 15 2012We demonstrate that any self-adjoint coupling in a quantum graph vertex can be approximated by a family of magnetic Schroedinger operators on a tubular network built over the graph. If such a manifold has a boundary, Neumann conditions are imposed at ... More
Operator estimates for the crushed ice problemOct 09 2017Dec 26 2017Let $\Delta_{\Omega_\varepsilon}$ be the Dirichlet Laplacian in the domain $\Omega_\varepsilon:=\Omega\setminus\left(\cup_i D_{i \varepsilon}\right)$. Here $\Omega\subset\mathbb{R}^n$ and $\{D_{i \varepsilon}\}_{i}$ is a family of tiny identical holes ... More
Path integrals and the essential self-adjointness of differential operators on noncompact manifoldsJun 28 2012Dec 07 2012We consider Schr\"odinger operators on possibly noncompact Riemannian manifolds, acting on sections in vector bundles, with locally square integrable potentials whose negative part is in the underlying Kato class. Using path integral methods, we prove ... More
Eigenvalue bracketing for discrete and metric graphsApr 07 2008We develop eigenvalue estimates for the Laplacians on discrete and metric graphs using different types of boundary conditions at the vertices of the metric graph. Via an explicit correspondence of the equilateral metric and discrete graph spectrum (also ... More
Asymptotic behaviour of the Hodge Laplacian spectrum on graph-like manifoldsFeb 10 2015We consider a family of compact, oriented and connected Riemannian manifolds shrinking to a metric graph and describe the asymptotic behaviour of the eigenvalues of the Hodge Laplacian. We apply our results to produce manifolds with spectral gaps of arbitrarily ... More
Locality of the heat kernel on metric measure spacesFeb 10 2017Nov 07 2017We will discuss what it means for a general heat kernel on a metric measure space to be local. We show that the Wiener measure associated to Brownian motion is local. Next we show that locality of the Wiener measure plus a suitable decay bound of the ... More
Wildly perturbed manifolds: norm resolvent and spectral convergenceFeb 04 2018Since the publication of the important work of Rauch and Taylor (Potential and scattering theory on wildly perturbed domains, JFA, 1975) a lot has been done to analyse wild perturbations of the Laplace operator. Here we present results concerning the ... More
On Open Scattering Channels for a Branched Covering of the Euclidean PlaneDec 26 2017We study the interaction of two scattering channels for a simple geometric model consisting in a double covering of the plane with two branch points, equipped with the Euclidean metric. We show that the scattering channels are open in the sense of arXiv:1202.0333 ... More
Approximation of quantum graph vertex couplings by scaled Schrödinger operators on thin branched manifoldsNov 22 2008We discuss approximations of vertex couplings of quantum graphs using families of thin branched manifolds. We show that if a Neumann type Laplacian on such manifolds is amended by suitable potentials, the resulting Schr\"odinger operators can approximate ... More
Spectral Gaps for Periodic Elliptic Operators with High Contrast: an OverviewJul 16 2002We discuss the band-gap structure and the integrated density of states for periodic elliptic operators in the Hilbert space $L_2(\R^m)$, for $m \ge 2$. We specifically consider situations where high contrast in the coefficients leads to weak coupling ... More
Generating spectral gaps by geometryJun 15 2004Jun 23 2005Motivated by the analysis of Schr\"odinger operators with periodic potentials we consider the following abstract situation: Let $\Delta_X$ be the Laplacian on a non-compact Riemannian covering manifold $X$ with a discrete isometric group $\Gamma$ acting ... More
On the spectra of carbon nano-structuresDec 07 2006Jan 19 2007An explicit derivation of dispersion relations and spectra for periodic Schr\"{o}dinger operators on carbon nano-structures (including graphen and all types of single-wall nano-tubes) is provided.
Existence of spectral gaps, covering manifolds and residually finite groupsMar 03 2005Dec 10 2007In the present paper we consider Riemannian coverings $(X,g) \to (M,g)$ with residually finite covering group $\Gamma$ and compact base space $(M,g)$. In particular, we give two general procedures resulting in a family of deformed coverings $(X,g_\eps) ... More
Anderson Localization for radial tree-like random quantum graphsNov 10 2006Jun 16 2008We prove that certain random models associated with radial, tree-like, rooted quantum graphs exhibit Anderson localization at all energies. The two main examples are the random length model (RLM) and the random Kirchhoff model (RKM). In the RLM, the lengths ... More
On Open Scattering Channels for Manifolds with EndsFeb 02 2012Nov 15 2013In the framework of time-dependent geometric scattering theory, we study the existence and completeness of the wave operators for perturbations of the Riemannian metric for the Laplacian on a complete manifold of dimension $n$. The smallness condition ... More
Gaps in the differential forms spectrum on cyclic coveringsAug 29 2007Apr 18 2008We are interested in the spectrum of the Hodge-de Rham operator on a cyclic covering $X$ over a compact manifold $M$ of dimension $n+1$. Let $\Sigma$ be a hypersurface in $M$ which does not disconnect $M$ and such that $M-\Sigma$ is a fundamental domain ... More
Convergence of sectorial operators on varying Hilbert spaceJul 22 2010Nov 14 2012Convergence of operators acting on a given Hilbert space is an old and well studied topic in operator theory. The idea of introducing a related notion for operators acting on arying spaces is natural. However, it seems that the first results in this direction ... More
Continuity of the integrated density of states on random length metric graphsNov 27 2008Apr 25 2009We establish several properties of the integrated density of states for random quantum graphs: Under appropriate ergodicity and amenability assumptions, the integrated density of states can be defined using an exhaustion procedure by compact subgraphs. ... More
Frustration index and Cheeger inequalities for discrete and continuous magnetic LaplaciansFeb 23 2015Aug 14 2015We discuss a Cheeger constant as a mixture of the frustration index and the expansion rate, and prove the related Cheeger inequalities and higher order Cheeger inequalities for graph Laplacians with cyclic signatures, discrete magnetic Laplacians on finite ... More
Continuity properties of the integrated density of states on manifoldsMay 08 2007We first analyze the integrated density of states (IDS) of periodic Schr\"odinger operators on an amenable covering manifold. A criterion for the continuity of the IDS at a prescribed energy is given along with examples of operators with both continuous ... More
A higher rank extension of the Askey-Wilson AlgebraMay 03 2017Oct 31 2017A novel generalization of the Askey-Wilson algebra is presented and shown to be associated with coproducts in the quantum algebra $U_q(su(1,1))$. This algebra has 15 non-commuting generators given by $Q^{(A)}$, with $A\subset \{1,2,3,4\}$ and their 5 ... More
QCD sum rules at finite density in the large-N_c limit: The coupling of the rho-nucleon system to the D_{13}(1520)Feb 13 2004QCD sum rules are studied for the vector-isovector current at finite baryon density in the limit of large number of colors N_c. For the condensate side it is shown that in this limit the four-quark condensate factorizes also for the finite density case. ... More
On factorizations in perturbative quantum gravityFeb 28 2011Apr 25 2011Some features of Einstein gravity are most easily understood from string theory but are not manifest at the level of the usual Lagrangian formulation. One example is the factorization of gravity amplitudes into gauge theory amplitudes. Based on the recently ... More
SUSY CP phases and asymmetries at collidersApr 21 2009In the Minimal Supersymmetric Standard Model, physical phases of complex parameters lead to CP violation. We show how triple products of particle momenta or spins can be used to construct asymmetries, that allow us to probe these CP phases. To give specific ... More
N=4 Mechanics, WDVV Equations and PolytopesNov 03 2008Nov 17 2008N=4 superconformal n-particle quantum mechanics on the real line is governed by two prepotentials, U and F, which obey a system of partial nonlinear differential equations generalizing the Witten-Dijkgraaf-Verlinde-Verlinde (WDVV) equation for F. The ... More
Gravitational lensingSep 25 2007This is a short and biased review of gravitational lensing with emphasis on the radio and especially VLBI aspects. We briefly explain the basic idea and give a short sketch of the discovery of the first lens, before we more systematically discuss the ... More
Emergent Probabilities in Quantum MechanicsMar 22 2006The transition from the quantum to the classical is governed by randomizing devices (RD), i.e., dynamical systems that are very sensitive to the environment. We show that, in the presence of RDs, the usual arguments based on the linearity of quantum mechanics ... More
Classicality in Quantum MechanicsNov 07 2006In this article we propose a solution to the measurement problem in quantum mechanics. We point out that the measurement problem can be traced to an a priori notion of classicality in the formulation of quantum mechanics. If this notion of classicality ... More
On the infinite-dimensional spin-2 symmetries in Kaluza-Klein theoriesNov 16 2005Jan 25 2006We consider the couplings of an infinite number of spin-2 fields to gravity appearing in Kaluza-Klein theories. They are constructed as the broken phase of a massless theory possessing an infinite-dimensional spin-2 symmetry. Focusing on a circle compactification ... More
Multifractality in a broad class of disordered systemsSep 21 2004We study multifractality in a broad class of disordered systems which includes, e.g., the diluted x-y model. Using renormalized field theory we analyze the scaling behavior of cumulant averaged dynamical variables (in case of the x-y model the angles ... More
Invariants of 3-manifolds from representations of the framed-tangle categoryMar 03 2002A new class of 3-manifold invariants is constructed from representations of the category of framed tangles.
The Gamma-Ray Properties of Unidentified EGRET SourcesFeb 28 2001Although the majority of gamma-ray sources still remain unidentified, we have various kinds of information to characterize the observational properties of unidentified EGRET sources. Despite astronomical properties like locations of individual sources ... More
The Self-Dual Critical N=2 StringJul 13 1996I review the covariant quantization of the closed fermionic string with (2,2) extended world-sheet supersymmetry on R^{2,2}. Results on n-point scattering amplitudes are presented, for tree- and one-loop world-sheets with arbitrary Maxwell instanton number. ... More
Integration Measure and Spectral Flow in the Critical N=2 StringDec 22 1995I present the moduli space of the (2+2)-dimensional critical closed fermionic string with two world-sheet supersymmetries. The integration of fermionic and Maxwell moduli in the presence of punctures yields the string measure for n-point amplitudes at ... More
Semiclassical Approach to Finite-N Matrix ModelsDec 17 1991Dec 18 1991We reformulate the zero-dimensional hermitean one-matrix model as a (nonlocal) collective field theory, for finite~$N$. The Jacobian arising by changing variables from matrix eigenvalues to their density distribution is treated {\it exactly\/}. The semiclassical ... More
A proof of Thorne's Hoop Conjecture for Einstein-Maxwell TheoryJul 18 2016Aug 18 2016We prove different variants of known singularity theorems and finally a statement in the spirit of Thorne's Hoop conjecture, concluding the existence of a region of finite lifetime that includes the outer trapped surface, and of a horizon. Conformal extensions ... More
Beyond information: A bit of meaningNov 06 2013Is our world just information? We argue that our current notion of information has one serious shortcoming: It is quite literally meaningless. We suggest a meaningful extension of the notion of information that is dynamic, internal, approximate, contains ... More
Instantons and Chern-Simons flows in 6, 7 and 8 dimensionsJan 30 2012Feb 23 2012The existence of K-instantons on a cylinder M^7 = R_tau x K/H over a homogeneous nearly K"ahler 6-manifold K/H requires a conformally parallel or a cocalibrated G_2-structure on M^7. The generalized anti-self-duality on M^7 implies a Chern-Simons flow ... More
Metrizability of spaces of homomorphisms between metric vector spacesMay 22 2009Nov 29 2014This note tries to give an answer to the following question: Is there a sufficiently rich class of metric vector spaces such that sufficiently large spaces of continuous linear maps between them are metrizable?
Imaging pulsar echoes at low frequenciesApr 25 2019Interstellar scattering is known to broaden distant objects spatially and temporally. The latter aspect is difficult to analyse, unless the signals carry their own time stamps. Pulsars are so kind to do us this favour. Typically the signature is a broadened ... More
Which spacetimes admit conformal compactifications?Sep 29 2014Nov 21 2014We consider the future causal boundary as a tool to find obstructions to conformal extensions, the latter being a slight generalization to conformal compactifications.
An infinite family of superintegrable deformations of the Coulomb potentialMar 26 2010May 11 2010We introduce a new family of Hamiltonians with a deformed Kepler- Coulomb potential dependent on an indexing parameter k. We show that this family is superintegrable for all rational k and compute the classical trajectories and quantum wave functions. ... More
An Oblivious O(1)-Approximation for Single Source Buy-at-BulkAug 26 2009We consider the single-source (or single-sink) buy-at-bulk problem with an unknown concave cost function. We want to route a set of demands along a graph to or from a designated root node, and the cost of routing x units of flow along an edge is proportional ... More
The optimal constants in Khintchine's inequality for the case 2<p<3Jan 28 2016A mean step in Haagerup's proof for the optimal constants in Khintchine's inequality is to show integral inequalities of type $\int(g^s-f^s)\mathrm{d}\mu\geq 0$. F.L. Nazarov and A.N. Podkorytov made Haagerup's proof much more clearer for the case 0<p<2 ... More
On Asteroid EngineeringNov 23 2015I pose the question of maximal Newtonian surface gravity on a homogeneous body of a given mass and volume but with variable shape. In other words, given an amount of malleable material of uniform density, how should one shape it in order for a microscopic ... More
Reconciliation of RDF* and Property GraphsSep 11 2014Nov 13 2014Both the notion of Property Graphs (PG) and the Resource Description Framework (RDF) are commonly used models for representing graph-shaped data. While there exist some system-specific solutions to convert data from one model to the other, these solutions ... More
SPARQL for a Web of Linked Data: Semantics and Computability (Extended Version)Mar 07 2012Apr 06 2012The World Wide Web currently evolves into a Web of Linked Data where content providers publish and link data as they have done with hypertext for the last 20 years. While the declarative query language SPARQL is the de facto for querying a-priory defined ... More
Long baseline experiments with LOFARAug 25 2010I present first results of LOFAR observations with international baselines. An important cornerstone was the detection of the first long-baseline fringes. Their analysis turns out to be extremely useful to investigate and solve a number of technical issues ... More
Why things fallOct 23 2007May 20 2008In this paper we discuss Internal Relativity, a recent program to address the problem of quantum gravity. In our approach we change the relationship between spacetime and matter. Currently we view matter as propagating on spacetime. Einstein's equations ... More
Anomalous elasticity in nematic and smectic elastomer tubuleAug 12 2008We study anomalous elasticity in the tubule phases of nematic and smectic elastomer membranes, which are flat in one direction and crumpled in another. These phases share the same macroscopic symmetry properties including spontaneously-broken in-plane ... More
Early Universe Cosmology in Internal RelativityMay 23 2008We present a new approach to early universe cosmology. Inflation is replaced by a phase transition in which both matter and geometry are created simultaneously. We calculate the spectrum of metric perturbations and show that it is flat. We then argue ... More
Emergent General RelativityApr 18 2006We review different approaches to quantum gravity in which spacetime is emerging. We discuss in some detail the proposals by G. Volovik and S. Lloyd and show how they differ in the way they treat time. We further propose an approach to quantum gravity ... More
On The Origin Of Unidentified EGRET Gamma-Ray SourcesJun 15 2005The identification of celestial gamma-ray sources with astronomical objects or object classes has remained the initial and most fundamental key for understanding their physical nature. The observational characteristic of a gamma-ray emitter and the conditions ... More
Quasilinear Drift Of Cosmic Rays In Weak Turbulent Electromagnetic FieldsJan 13 2005A general quasilinear transport parameter for particle drift in arbitrary turbulence geometry is presented. The new drift coefficient is solely characterized by a nonresonant term and is evaluated for slab and two-dimensional turbulence geometry. The ... More
Quasinormal Modes, the Area Spectrum, and Black Hole EntropyNov 24 2002The results of canonical quantum gravity concerning geometric operators and black hole entropy are beset by an ambiguity labelled by the Immirzi parameter. We use a result from classical gravity concerning the quasinormal mode spectrum of a black hole ... More
From N=2 Fermionic Strings to Superstrings?Dec 31 1994I review the covariant quantization of the critical $N{=}2$ fermionic string with and without a global ${\bf Z}_2$ twist. The BRST analysis yields massless bosonic and fermionic vertex operators in various ghost and picture number sectors, as well as ... More
Continuum estimate of the heavy quark momentum diffusion coefficient $κ$Sep 12 2014Among quantities playing a central role in the theoretical interpretation of heavy ion collision experiments at RHIC and LHC are so-called transport coefficients. Out of those heavy quark diffusion coefficients play an important role e.g. for the analysis ... More
2-universality in randomly perturbed graphsFeb 05 2019A graph $G$ is called universal for a family of graphs $\mathcal{F}$ if it contains every element $F \in \mathcal{F}$ as a subgraph. Let $\mathcal{F}(n,2)$ be the family of all graphs with maximum degree $2$. Ferber, Kronenberg, and Luh [Embedding large ... More
A note on $H^* (\rm{Emb}_0 (M,N))$Jan 25 2002Jan 29 2002Aim of this note is to gain cohomological information about the infinite-dimensional manifold $\rm{Emb}_0 (M,N)$ of asymptotically fixed embeddings of M into N from the topology of the target manifold N. This paper has been withdrawn by the author due ... More
A metric approach to Fréchet geometryDec 14 2006Jun 16 2007The aim of this article is to present the category of bounded Frechet manifolds in respect to which we will review the geometry of Frechet manifolds with a stronger accent on its metric aspect. An inverse function theorem in the sense of Nash and Moser ... More
On the von Neumann rule in quantizationMar 25 2019Apr 15 2019We show that any linear quantization map into the space of self-adjoint operators in a Hilbert space violates the von Neumann rule on post-composition with real functions.
Gate capacitance of back-gated nanowire field-effect transistorsJul 14 2006Gate capacitances of back-gated nanowire field-effect transistors (NW-FETs) are calculated by means of finite element methods and the results are compared with analytical results of the ``metallic cylinder on an infinite metal plate model''. Completely ... More
A note on closed isometric embeddingsMay 30 2008May 20 2016This note is about a little extension of Nash's embedding theorem in the case of complete manifolds.
A note on invariant temporal functionsFeb 09 2015Jul 18 2016The purpose of this article is to present a result on the existence of Cauchy temporal functions invariant by the action of a compact group of conformal transformations in arbitrary globally hyperbolic manifolds. Moreover, the previous results about the ... More
Special temporal functions on globally hyperbolic manifoldsApr 09 2009Oct 22 2012In this article, existence results concerning temporal functions with additional properties on a globally hyperbolic manifold are obtained. These properties are certain bounds on geometric quantities as lapse and shift. The results are linked to completeness ... More
Black holes in Einstein-Maxwell TheoryJul 18 2016Oct 06 2018We prove variants of known singularity theorems ensuring the existence of a region of finite lifetime that are particularly well applicable if the solution admits a conformal extension, a property satisfied e.g. by maximal Cauchy developments of Einstein-Maxwell ... More
Consideration of the restriction of lateral contraction in the elastic behaviour of cohesive zone modelsNov 06 2015Cohesive zone models do not consider the lateral contraction of adhesive layers under tensile loads. The constraint of the lateral contraction by the adherents which depends on the geometry of the adhesive layer has a major influence on the normal stiffness ... More
New results on affine invariant pointsSep 20 2015Apr 04 2016We prove a conjecture of B. Gr\"unbaum stating that the set of affine invariant points of a convex body equals to the set of points invariant under all affine linear symmetries of the convex body. As a consequence we give a short proof on the fact that ... More
Internal RelativityMar 12 2012General relativity differs from other forces in nature in that it can be made to disappear locally. This is the essence of the equivalence principle. In general relativity the equivalence principle is implemented using differential geometry. The connection ... More
Time is not the problemApr 22 2009Attempts to quantize general relativity encounter an odd problem. The Hamiltonian that normally generates time evolution vanishes in the case of general relativity as a result of diffeomorphism invariance. The theory seems to be saying that time does ... More
T-duality versus Gauge SymmetryJan 18 2011We review the recently constructed `double field theory' which introduces in addition to the conventional coordinates associated to momentum modes coordinates associated to winding modes. Thereby, T-duality becomes a global symmetry of the theory, which ... More
How light can the lightest neutralino be?Nov 15 2010In this talk we summarize previous work on mass bounds of a light neutralino in the Minimal Supersymmetric Standard Model. We show that without the GUT relation between the gaugino mass parameters M_1 and M_2, the mass of the lightest neutralino is essentially ... More
Dissertation: The Cauchy Problem for MembranesJul 16 2008Aug 10 2008We show existence and uniqueness for timelike minimal submanifolds in ambient Lorentz manifolds admitting a time function. The initial value formulation introduced and the conditions imposed on the initial data are given in purely geometric terms.
Supersymmetric noncommutative solitonsJul 24 2007I consider a supersymmetric Bogomolny-type model in 2+1 dimensions originating from topological string theory. By a gauge fixing this model is reduced to a supersymmetric U(n) chiral model with a Wess-Zumino-Witten-type term in 2+1 dimensions. After a ... More
Smectic elastomer membranesApr 24 2007We present a model for smectic elastomer membranes which includes elastic and liquid crystalline degrees of freedom. Based on our model, we determined the qualitative phase diagram of a smectic elastomer membrane using mean-field theory. This phase diagram ... More
Massive Kaluza-Klein Theories and their Spontaneously Broken SymmetriesDec 21 2006In this thesis we investigate the effective actions for massive Kaluza-Klein states, focusing on the massive modes of spin-3/2 and spin-2 fields. To this end we determine the spontaneously broken gauge symmetries associated to these `higher spin' states ... More
The Cauchy Problem for MembranesJul 22 2008We show existence and uniqueness for timelike minimal submanifolds (world volume of p-branes) in ambient Lorentz manifolds admitting a time function in a neighborhood of the initial submanifold. The initial value formulation introduced and the conditions ... More
WDVV solutions from orthocentric polytopes and Veselov systemsMay 21 2008Jun 26 2008N=4 superconformal n-particle quantum mechanics on the real line is governed by two prepotentials, U and F, which obey a system of partial nonlinear differential equations generalizing the Witten-Dijkgraaf-Verlinde-Verlinde (WDVV) equation. For U=0 one ... More
New Hints from General RelativityJan 10 2004The search for a quantum theory of gravity has followed two parallel but different paths. One aims at arriving at the final theory starting from a priori assumptions as to its form and building it from the ground up. The other tries to infer as much as ... More
LensCLEANing JVAS B0218+357 to determine H0Dec 13 2001We use the radio ring in JVAS B0218+357 to constrain the mass models with LensCLEAN. H0 is determined from the resulting model and the time delay. B0218+357 is one of the best systems to reduce systematic errors in H0 and may thus be called a "golden ... More
Mathematics and Physics of N=2 StringsDec 30 1999Open and closed strings with two worldsheet supersymmetries in 2+2 dimensional spacetime are reviewed in the NSR formulation. I briefly discuss their quantization, mutual and self-interactions, classical spacetime dynamics and interpretation in terms ... More
On the BRST Cohomology of N=2 StringsAug 21 1995We analyze the BRST cohomology of the critical N=2 NSR string using chiral bosonization. Picture-changing and spectral flow is made explicit in a holomorphic field basis. The integration of fermionic and U(1) moduli is performed and yields picture- and ... More
On the von Neumann rule in quantizationMar 25 2019We show that any linear quantization map into the space of self-adjoint operators in a Hilbert space violates the von Neumann rule about post-composition with real functions.
HorizonsNov 19 2011Aug 13 2015We define different notions of black holes, event horizons and Killing horizons for a general time-oriented manifold $(M,g)$ extending previous notions but without the assumption of asymptotical flatness. The notions of 'horizon' are always conformally ... More