Results for "Jan Sieber"

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Optimization along Families of Periodic and Quasiperiodic Orbits in Dynamical Systems with DelayJan 26 2019This paper generalizes a previously-conceived, continuation-based optimization technique for scalar objective functions on constraint manifolds to cases of periodic and quasiperiodic solutions of delay-differential equations. A Lagrange formalism is used ... More
Longtime behavior of coupled wave equations for semiconductor lasersAug 09 2013Coupled wave equations are popular tool for investigating longitudinal dynamical effects in semiconductor lasers, for example, sensitivity to delayed optical feedback. We study a model that consists of a hyperbolic linear system of partial differential ... More
Finding periodic orbits in state-dependent delay differential equations as roots of algebraic equationsOct 12 2010Feb 26 2015In this paper we prove that periodic boundary-value problems (BVPs) for delay differential equations are locally equivalent to finite-dimensional algebraic systems of equations. We rely only on regularity assumptions that follow those of the review by ... More
Generic stabilizability for time-delayed feedback controlAug 23 2015Apr 24 2016Time delayed feedback control is one of the most successful methods to discover dynamically unstable features of a dynamical system in an experiment. This approach feeds back only terms that depend on the difference between the current output and the ... More
DDE-BIFTOOL Manual - Bifurcation analysis of delay differential equationsJun 27 2014Sep 06 2016DDEBIFTOOL is a collection of Matlab routines for numerical bifurcation analysis of systems of delay differential equations with discrete constant and state-dependent delays. The package supports continuation and stability analysis of steady state solutions ... More
Characteristic matrices for linear periodic delay differential equationsMay 25 2010Jan 21 2011Szalai et al. (SIAM J. on Sci. Comp. 28(4), 2006) gave a general construction for characteristic matrices for systems of linear delay-differential equations with periodic coefficients. First, we show that matrices constructed in this way can have a discrete ... More
Relative equilibria and relative periodic solutions in systems with time-delay and $S^{1}$ symmetryJun 14 2013Dec 03 2016We study properties of basic solutions in systems with dime delays and $S^1$-symmetry. Such basic solutions are relative equilibria (CW solutions) and relative periodic solutions (MW solutions). It follows from the previous theory that the number of CW ... More
Early-warning indicators for rate-induced tippingSep 05 2015Sep 23 2016A dynamical system is said to undergo rate-induced tipping when it fails to track its quasi-equilibrium state due to an above-critical-rate change of system parameters. We study a prototypical model for rate-induced tipping, the saddle-node normal form ... More
Probability of noise and rate-induced tippingJun 27 2016We propose a formula to approximate the probability of rate-induced tipping with additive white noise occurring for small to moderate equilibrium drift speeds. Early-warning indicators have generally been used on historical tipping events as a form of ... More
Small-scale instabilities in dynamical systems with slidingNov 12 2008Jul 12 2010We demonstrate with a minimal example that in Filippov systems (dynamical systems governed by discontinuous but piecewise smooth vector fields) stable periodic motion with sliding is not robust with respect to stable singular perturbations. We consider ... More
Relative equailibria and relative periodic solutions in systems with time-delay and $S^{1}$ symmetryJun 14 2013We study properties of basic solutions in systems with dime delays and $S^1$-symmetry. Such basic solutions are relative equilibria (CW solutions) and relative periodic solutions (MW solutions). It follows from the previous theory that the number of CW ... More
Using feedback control and Newton iterations to track dynamically unstable phenomena in experimentsMar 18 2009If one wants to explore the properties of a dynamical system systematically one has to be able to track equilibria and periodic orbits regardless of their stability. If the dynamical system is a controllable experiment then one approach is a combination ... More
Early-warning indicators in the dynamic regimeSep 23 2016Early-warning indicators (increase of autocorrelation and variance) are commonly applied to time series data to try and detect tipping points of real-world systems. The theory behind these indicators originates from approximating the fluctuations around ... More
Bifurcation analysis of delay-induced resonances of the El-Nino Southern OscillationSep 13 2011May 28 2014Models of global climate phenomena of low to intermediate complexity are very useful for providing an understanding at a conceptual level. An important aspect of such models is the presence of a number of feedback loops that feature considerable delay ... More
On the Stability of a Chain of Phase OscillatorsJan 25 2011Jun 02 2011We study a chain of $N+1$ phase oscillators with asymmetric but uniform coupling. This type of chain possesses $2^{N}$ ways to synchronize in so-called travelling wave states, i.e. states where the phases of the single oscillators are in relative equilibrium. ... More
Regular and irregular patterns of self-localized excitation in arrays of coupled phase oscillatorsJan 30 2015Apr 24 2015We study a system of phase oscillators with nonlocal coupling in a ring that supports self-organized patterns of coherence and incoherence, called chimera states. Introducing a global feedback loop, connecting the phase lag to the order parameter, we ... More
Nonlinear softening as a predictive precursor to climate tippingMar 15 2011Dec 09 2011Approaching a dangerous bifurcation, from which a dynamical system such as the Earth's climate will jump (tip) to a different state, the current stable state lies within a shrinking basin of attraction. Persistence of the state becomes increasingly precarious ... More
Climate tipping as a noisy bifurcation: a predictive techniqueJul 08 2010Nov 30 2010It is often known, from modelling studies, that a certain mode of climate tipping (of the oceanic thermohaline circulation, for example) is governed by an underlying fold bifurcation. For such a case we present a scheme of analysis that determines the ... More
Convergence of equation-free methods in the case of finite time scale separation with application to deterministic and stochastic systemsJan 31 2017Sep 12 2018A common approach to studying high-dimensional systems with emergent low-dimensional behavior is based on lift-evolve-restrict maps (called equation-free methods): first, a user-defined lifting operator maps a set of low-dimensional coordinates into the ... More
Controlling unstable chaos: Stabilizing chimera states by feedbackOct 28 2013Dec 18 2013We present a control scheme that is able to find and stabilize an unstable chaotic regime in a system with a large number of interacting particles. This allows us to track a high dimensional chaotic attractor through a bifurcation where it loses its attractivity. ... More
Systematic experimental exploration of bifurcations with non-invasive controlSep 17 2012Mar 15 2013We present a general method for systematically investigating the dynamics and bifurcations of a physical nonlinear experiment. In particular, we show how the odd-number limitation inherent in popular non-invasive control schemes, such as (Pyragas) time-delayed ... More
Shock-Sensitivity in Shell-Like Structures: with Simulations of Spherical Shell BucklingSep 20 2015Oct 06 2015Under increasing compression, an unbuckled shell is in a metastable state which becomes increasingly precarious as the buckling load is approached. So to induce premature buckling a lateral disturbance will have to overcome a decreasing energy barrier ... More
Nonlinear dynamic interactions between flow-induced galloping and shell-like bucklingJan 10 2014For an elastic system that is non-conservative but autonomous, subjected for example to time-independent loading by a steadily flowing fluid (air or water), a dangerous bifurcation, such as a sub-critical bifurcation, or a cyclic fold, will trigger a ... More
Evolution of the L1 halo family in the radial solar sail CRTBPAug 15 2013Jul 28 2014We present a detailed investigation of the dramatic changes that occur in the $\mathcal{L}_1$ halo family when radiation pressure is introduced into the Sun-Earth circular restricted three-body problem (CRTBP). This photo-gravitational CRTBP can be used ... More
Introduction to Multi-Agent SimulationMar 27 2008When designing systems that are complex, dynamic and stochastic in nature, simulation is generally recognised as one of the best design support technologies, and a valuable aid in the strategic and tactical decision making process. A simulation model ... More
On the stability of periodic orbits in delay equations with large delayJan 06 2011Dec 05 2012We prove a necessary and sufficient criterion for the exponential stability of periodic solutions of delay differential equations with large delay. We show that for sufficiently large delay the Floquet spectrum near criticality is characterized by a set ... More
Temporal dissipative solitons in time-delay feedback systemsJan 11 2019Localized states are a universal phenomenon observed in spatially distributed dissipative nonlinear systems. Known as dissipative solitons, auto-solitons, spot or pulse solutions, these states play an important role in data transmission using optical ... More
Extremum seeking via continuation techniques for optimizing biogas production in the chemostatFeb 13 2013May 13 2013We consider the chemostat model with the substrate concentration as the single measurement. We propose a control strategy that drives the system at a steady state maximizing the gas production without the knowledge of the specific growth rate. Our approach ... More
Equation-Free Analysis of Macroscopic Behavior in Traffic and Pedestrian FlowFeb 03 2014Equation-free methods make possible an analysis of the evolution of a few coarse-grained or macroscopic quantities for a detailed and realistic model with a large number of fine-grained or microscopic variables, even though no equations are explicitly ... More
Temporal dissipative solitons in time-delay feedback systemsJan 11 2019Apr 08 2019Localized states are a universal phenomenon observed in spatially distributed dissipative nonlinear systems. Known as dissipative solitons, auto-solitons, spot or pulse solutions, these states play an important role in data transmission using optical ... More
Implicit Methods for Equation-Free Analysis: Convergence Results and Analysis of Emergent Waves in Microscopic Traffic ModelsJan 25 2013Jul 09 2014We introduce a general formulation for an implicit equation-free method in the setting of slow-fast systems. First, we give a rigorous convergence result for equation-free analysis showing that the implicitly defined coarse-level time stepper converges ... More
A method for the reconstruction of unknown non-monotonic growth functions in the chemostatAug 08 2012Jan 10 2013We propose an adaptive control law that allows one to identify unstable steady states of the open-loop system in the single-species chemostat model without the knowledge of the growth function. We then show how one can use this control law to trace out ... More
Probing shells against buckling: a non-destructive technique for laboratory testingDec 13 2017This paper addresses testing of compressed structures, such as shells, that exhibit catastrophic buckling and notorious imperfection sensitivity. The central concept is the probing of a loaded structural specimen by a controlled lateral displacement to ... More
Effects of periodic forcing on a Paleoclimate delay modelAug 07 2018Jan 11 2019We present a study of a delay differential equation (DDE) model for the Mid-Pleistocene Transition (MPT). We investigate the behavior of the model when subjected to periodic forcing. The unforced model has a bistable region consisting of a stable equilibrium ... More
Nonlinear Dynamics of Spherical Shells Buckling under Step PressureDec 16 2018Feb 06 2019Dynamic buckling is addressed for complete elastic spherical shells subject to a rapidly applied step in external pressure. Insights from the perspective of nonlinear dynamics reveal essential mathematical features of the buckling phenomena. To capture ... More
Modelling Office Energy Consumption: An Agent Based ApproachJul 20 2016In this paper, we develop an agent-based model which integrates four important elements, i.e. organisational energy management policies/regulations, energy management technologies, electric appliances and equipment, and human behaviour, based on a case ... More
Modelling Electricity Consumption in Office Buildings: An Agent Based ApproachMay 31 2013In this paper, we develop an agent-based model which integrates four important elements, i.e. organisational energy management policies/regulations, energy management technologies, electric appliances and equipment, and human behaviour, to simulate the ... More
Optimal top dag compressionDec 15 2017It is shown that for a given ordered node-labelled tree of size $n$ and with $s$ many different node labels, one can construct in linear time a top dag of height $O(\log n)$ and size $O(n / \log_\sigma n) \cap O(d \cdot \log n)$, where $\sigma = \max\{ ... More
The Mid-Pleistocene Transition induced by delayed feedback and bistabilityDec 20 2017The Mid-Pleistocene Transition, the shift from 41 kyr to 100 kyr glacial-interglacial cycles that occurred roughly 1 Myr ago, is often considered as a change in internal climate dynamics. Here we revisit the model of Quaternary climate dynamics that was ... More
Derivation of Delay Equation Climate Models Using the Mori-Zwanzig FormalismFeb 08 2019Models incorporating delay have been frequently used to understand climate variability phenomena, but often the delay is introduced through an ad-hoc physical reasoning, such as the propagation time of waves. In this paper, the Mori-Zwanzig formalism ... More
The Mid-Pleistocene Transition induced by delayed feedback and bistabilityDec 20 2017Nov 09 2018The Mid-Pleistocene Transition, the shift from 41 kyr to 100 kyr glacial-interglacial cycles that occurred roughly 1 Myr ago, is often considered as a change in internal climate dynamics. Here we revisit the model of Quaternary climate dynamics that was ... More
Comparing Decison Support Tools for Cargo Screening ProcessesJul 02 2013When planning to change operations at ports there are two key stake holders with very different interests involved in the decision making processes. Port operators are attentive to their standards, a smooth service flow and economic viability while border ... More
Evaluating Different Cost-Benefit Analysis Methods for Port Security OperationsMay 31 2013Service industries, such as ports, are attentive to their standards, a smooth service flow and economic viability. Cost benefit analysis has proven itself as a useful tool to support this type of decision making; it has been used by businesses and governmental ... More
Semiclassical Transition from an Elliptical to an Oval BilliardNov 14 1996Semiclassical approximations often involve the use of stationary phase approximations. This method can be applied when $\hbar$ is small in comparison to relevant actions or action differences in the corresponding classical system. In many situations, ... More
Wavefunctions, Green's functions and expectation values in terms of spectral determinantsJun 27 2007We derive semiclassical approximations for wavefunctions, Green's functions and expectation values for classically chaotic quantum systems. Our method consists of applying singular and regular perturbations to quantum Hamiltonians. The wavefunctions, ... More
Spectral statistics in chaotic systems with a point interactionMar 08 2000Aug 14 2000We consider quantum systems with a chaotic classical limit that are perturbed by a point-like scatterer. The spectral form factor K(tau) for these systems is evaluated semiclassically in terms of periodic and diffractive orbits. It is shown for order ... More
Leading off-diagonal approximation for the spectral form factor for uniformly hyperbolic systemsSep 06 2002We consider the semiclassical approximation to the spectral form factor K(tau) for two-dimensional uniformly hyperbolic systems, and derive the first off-diagonal correction for small tau. The result agrees with the tau^2-term of the form factor for the ... More
Semiclassical Treatment of Diffraction in Billiard Systems with a Flux LineMar 31 1999In billiard systems with a flux line semiclassical approximations for the density of states contain contributions from periodic orbits as well as from diffractive orbits that are scattered on the flux line. We derive a semiclassical approximation for ... More
Geometrical theory of diffraction and spectral statisticsOct 06 1999We investigate the influence of diffraction on the statistics of energy levels in quantum systems with a chaotic classical limit. By applying the geometrical theory of diffraction we show that diffraction on singularities of the potential can lead to ... More
Billiard Systems in Three Dimensions: The Boundary Integral Equation and the Trace FormulaOct 31 1997We derive semiclassical contributions of periodic orbits from a boundary integral equation for three-dimensional billiard systems. We use an iterative method that keeps track of the composition of the stability matrix and the Maslov index as an orbit ... More
Modelling Electrical Car Diffusion Based on AgentsSep 02 2014Replacing traditional fossil fuel vehicles with innovative zero-emission vehicles for the transport in ci ties is one of the major tactics to achieve the UK government 2020 target of cutting emission. We are developing an agent-based simulation model ... More
Universality in quantum parametric correlationsMay 03 1999Jul 30 1999We investigate the universality of correlation functions of chaotic and disordered quantum systems as an external parameter is varied. A new, general scaling procedure is introduced which makes the theory invariant under reparametrizations. Under certain ... More
Experimental continuation of periodic orbits through a foldApr 02 2008Jun 12 2008We present a continuation method that enables one to track or continue branches of periodic orbits directly in an experiment when a parameter is changed. A control-based setup in combination with Newton iterations ensures that the periodic orbit can be ... More
Uniform Approximation for Period-Quadrupling BifurcationsAug 21 1997We derive a uniform approximation for semiclassical contributions of periodic orbits to the spectral density which is valid for generic period-quadrupling bifurcations in systems with a mixed phase space. These bifurcations involve three periodic orbits ... More
Semiclassical expansion of parametric correlation functions of the quantum time delayAug 24 2006We derive semiclassical periodic orbit expansions for a correlation function of the Wigner time delay. We consider the Fourier transform of the two-point correlation function, the form factor $K(\tau,x,y,M)$, that depends on the number of open channels ... More
Bifurcations of Periodic Orbits and Uniform ApproximationsJan 22 1997We derive uniform approximations for contributions to Gutzwiller's periodic-orbit sum for the spectral density which are valid close to bifurcations of periodic orbits in systems with mixed phase space. There, orbits lie close together and give collective ... More
Semiclassical universality of parametric spectral correlationsAug 07 2006We consider quantum systems with a chaotic classical limit that depend on an external parameter, and study correlations between the spectra at different parameter values. In particular, we consider the parametric spectral form factor $K(\tau,x)$ which ... More
Semiclassical Theory of Chaotic Quantum TransportMay 08 2002We present a refined semiclassical approach to the Landauer conductance and Kubo conductivity of clean chaotic mesoscopic systems. We demonstrate for systems with uniformly hyperbolic dynamics that including off-diagonal contributions to double sums over ... More
The semiclassical relation between open trajectories and periodic orbits for the Wigner time delayNov 28 2007The Wigner time delay of a classically chaotic quantum system can be expressed semiclassically either in terms of pairs of scattering trajectories that enter and leave the system or in terms of the periodic orbits trapped inside the system. We show how ... More
Particle creation and annihilation at interior boundaries: One-dimensional modelsNov 10 2015We describe creation and annihilation of particles at external sources in one spatial dimension in terms of interior-boundary conditions (IBCs). We derive explicit solutions for spectra, (generalised) eigenfunctions, as well as Green functions, spectral ... More
Singular compactness and definability for $Σ$-cotorsion and Gorenstein modulesApr 24 2018May 25 2018We introduce a general version of singular compactness theorem which makes it possible to show that being a $\Sigma$-cotorsion module is a property of the complete theory of the module. As an application of the powerful tools developed along the way, ... More
The countable Telescope Conjecture for module categoriesJan 25 2008May 16 2008By the Telescope Conjecture for Module Categories, we mean the following claim: "Let R be any ring and (A, B) be a hereditary cotorsion pair in Mod-R with A and B closed under direct limits. Then (A, B) is of finite type." We prove a modification of this ... More
A First Approach on Modelling Staff Proactiveness in Retail Simulation ModelsAug 15 2011There has been a noticeable shift in the relative composition of the industry in the developed countries in recent years; manufacturing is decreasing while the service sector is becoming more important. However, currently most simulation models for investigating ... More
On invariant Schreier structuresSep 20 2013May 30 2014Schreier graphs, which possess both a graph structure and a Schreier structure (an edge-labeling by the generators of a group), are objects of fundamental importance in group theory and geometry. We study the Schreier structures with which unlabeled graphs ... More
Blowup of Jang's equation at outermost marginally trapped surfacesNov 29 2007Aug 17 2009The aim of this paper is to collect some facts about the blowup of Jang's equation. First, we discuss how to construct solutions that blow up at an outermost MOTS. Second, we exclude the possibility that there are extra blowup surfaces in data sets with ... More
NLO QCD corrections to W+b W-antib production at hadron colliders: new developments and new issuesDec 13 2014This short summary reviews the recent progress in the computation of higher-order corrections to W+b W-antib production. In addition, new phenomenological studies reveal potential problems that may affect precision measurements at the LHC.
Functional calculus for $C_{0}$-groups using (co)typeAug 09 2015We study the functional calculus properties of generators of $C_{0}$-groups under type and cotype assumptions on the underlying Banach space. In particular, we show the following. Let $-iA$ generate a $C_{0}$-group on a Banach space $X$ with type $p\in[1,2]$ ... More
Quantization of GeometryNov 22 1994Jun 08 1995Contents: 1. Introduction 2. Bosonic propagators and random paths 3. Random surfaces and strings 4. Matrix models and two-dimensional quantum gravity 5. The mystery of $c > 1$ 6. Euclidean quantum gravity in $d > 2$ 7. Discussion
Barriers in Quantum GravityAug 23 1994I discuss recent progress in our understanding of two barriers in quantum gravity: $c > 1$ in the case of 2d quantum gravity and $D > 2$ in the case of Euclidean Einstein-Hilbert gravity formulated in space-time dimensions $D >2$.
Indirect detection of Dark matter with gamma-rays - status and perspectivesMar 29 2011In my contribution I review the status of indirect detection of dark matter with gamma-rays, including results of the Fermi Gamma-ray Space Telescope as well as Imaging Air Cherenkov Telescopes (IACT), like H.E.S.S., MAGIC and VERITAS. I will also briefly ... More
Indirect Detection of WIMP Dark Matter: a compact reviewNov 07 2014Indirect detection of dark matter particles, i.e. the detection of annihilation or decay products of Weakly Interacting Massive Particles, has entered a pivotal phase as experiments reach sensitivities that probe the most interesting parameter space. ... More
On sensitivity calculations for neutrino oscillation experimentsOct 16 2007Calculations of sensitivities of future experiments are a necessary ingredient in experimental high energy physics. Especially in the context of measurements of the neutrino oscillation parameters extensive studies are performed to arrive at the optimal ... More
Beam Test Performance and Simulation of Prototypes for the ALICE Silicon Pixel DetectorNov 07 2005The silicon pixel detector (SPD) of the ALICE experiment in preparation at the Large Hadron Collider (LHC) at CERN is designed to provide the precise vertex reconstruction needed for measuring heavy flavor production in heavy ion collisions at very high ... More
eROSITA - Nearby Young Stars in X-raysOct 05 2015X-ray surveys are well suited to detect, identify and study young stars based on their high levels of magnetic activity and thus X-ray brightness. The eROSITA instrument onboard the Spectrum-Roentgen-Gamma (SRG) satellite will perform an X-ray all-sky ... More
Geometry of Membrane Sigma ModelsDec 27 2015String theory still remains one of the promising candidates for a unification of the theory of gravity and quantum field theory. One of its essential parts is relativistic description of moving multi-dimensional objects called membranes (or p-branes) ... More
A rigid Urysohn-like spaceNov 30 2015Recall that the Rado graph is the unique countable graph that realizes all one-point extensions of its finite subgraphs. The Rado graph is well-known to be universal and homogeneous in the sense that every isomorphism between finite subgraphs of $R$ extends ... More
An equivalence between Frobenius algebras and Calabi-Yau categoriesSep 21 2016We show that the bigroupoid of separable symmetric Frobenius algebras over an algebraically closed field and the bigroupoid of finitely semi-simple Calabi-Yau categories are equivalent. To this end, we construct a trace on the category of finitely-generated ... More
The Higgs Physics Program at the International Linear ColliderNov 01 2015The International Linear Collider (ILC) is a proposed electron -- positron collider with a collision energy of $\sqrt{s}$ = 500 GeV in the baseline configuration. The ILC physics program takes full advantage of the fact that the machine can be operated ... More
Charginos and Neutralinos at e+e- Linear CollidersMay 16 2002It is shown how the fundamental gaugino and higgsino parameters of the chargino and neutralino system in the MSSM can completely be determined in high--precision experiments at e+e- linear colliders even if in their initial phase only the light charginos ... More
Supersymmetry at and beyond the LHCApr 17 2008Prospects for SUSY discoveries and measurements at future colliders LHC and ILC are discussed. The problem of reconstructing the underlying theory and SUSY breaking mechanism is also addressed.
Testing CP Violation and Universal Extra Dimensions at Future CollidersOct 08 2004A future linear e+e- collider with a clean environment, tunable collision energy, high luminosity, polarized incoming beams, and additional e-e-, e-gamma and gamma-gamma modes, will offer precision tools to explore new physics. Here we summarize three ... More
Supersymmetric $W$ Boson Decays as a Means to Search for Charginos and NeutralinosMar 26 1997If the sneutrino mass is below the chargino mass, the dominant decay mode of the lightest chargino is via a two-body decay channel $\chi^{\pm}_1 \ra \tilde{\nu} + l^{\pm}$. Sneutrinos are invisible in $R$-parity conserving supersymmetric models and, if ... More
On the Existence of a Maximal Cauchy Development for the Einstein Equations - a DezornificationSep 29 2013Jul 02 2015In 1969, Choquet-Bruhat and Geroch established the existence of a unique maximal globally hyperbolic Cauchy development of given initial data for the Einstein equations. Their proof, however, has the unsatisfactory feature that it relies crucially on ... More
Approximate invariance of metabolic energy per synapse during development in mammalian brainsApr 17 2012During mammalian development the cerebral metabolic rate correlates qualitatively with synaptogenesis, and both often exhibit bimodal temporal profiles. Despite these non-monotonic dependencies, it is found based on empirical data for different mammals ... More
The extended states in disordered 1D systems in the presence of the generalized $N$-mer correlationsJul 07 2016Aug 22 2016We have been investigating the problem of the Anderson localization in a disordered one dimensional tight-binding model. The disorder is created by the interaction of mobile particles with other species, immobilized at random positions. We introduce a ... More
Wall-crossing of D4-branes using flow treesMar 08 2010The moduli dependence of D4-branes on a Calabi-Yau manifold is studied using attractor flow trees, in the large volume limit of the Kahler cone. One of the moduli dependent existence criteria of flow trees is the positivity of the flow parameters along ... More
Stability and duality in N=2 supergravityJun 09 2009Aug 17 2010The BPS-spectrum is known to change when moduli cross a wall of marginal stability. This paper tests the compatibility of wall-crossing with S-duality and electric-magnetic duality for N=2 supergravity. To this end, the BPS-spectrum of D4-D2-D0 branes ... More
Time-optimal Control of Spin SystemsJan 19 2006The paper discusses various aspects of time-optimal control of quantum spin systems, modelled as right-invariant systems on a compact Lie group G. The main results are the reduction of such a system to an equivalent system on a homogeneous space G/H, ... More
A modern, but way too short history of the theory of superconductivity at a high temperatureDec 25 2010Jan 04 2011An attempt to shed light on the various belief/idea systems in high Tc superconductivity that are at present popular. This text is in first instance intended to serve both string theorists and junior condensed matter physicists who want to enter this ... More
Boltzmann entropy and the microcanonical ensembleDec 24 2004Boltzmann's entropy is slightly modified to make it suitable for discussing phase transitions in finite systems. As an example it is shown that the pendulum undergoes a second order phase transition when passing from a vibrational to a rotating state. ... More
On the maximum entropy principle in non-extensive thermostatisticsMay 21 2004It is possible to derive the maximum entropy principle from thermodynamic stability requirements. Using as a starting point the equilibrium probability distribution, currently used in non-extensive thermostatistics, it turns out that the relevant entropy ... More
Parameter estimation in nonextensive thermostatisticsSep 30 2005Equilibrium statistical physics is considered from the point of view of statistical estimation theory. This involves the notions of statistical model, of estimators, and of exponential family. A useful property of the latter is the existence of identities, ... More
Continuity of a class of entropies and relative entropiesAug 26 2002Jan 30 2004The present paper studies continuity of generalized entropy functions and relative entropies defined using the notion of a deformed logarithmic function. In particular, two distinct definitions of relative entropy are discussed. As an application, all ... More
Nuclear shadowing in the light-cone dipole approachMar 12 2008We study nuclear shadowing at small Bjorken x < 0.01 in the color dipole approach. Such a light-cone quantum-chromodynamics formalism based on the Green function technique incorporates naturally color transparency and coherence length effects. The nuclear ... More
Topological Quantum Field Theory and Pure Yang-Mills DynamicsAug 03 2004Aug 03 2006By considering specific limits in the gauge coupling constant of pure Yang--Mills dynamics, it is shown how there exist topological quantum field theory sectors in such systems defining nonperturbative topological configurations of the gauge fields which ... More
Positronium Spectroscopy in a Magnetic FieldAug 09 1993Hyperfine spectroscopy of positronium formed in the presence of a static magnetic field is considered. Generalising the situation hitherto developed in the literature, the magnetic field is not assumed to be parallel to the momentum of incoming polarised ... More
Finite Euler Hierarchies And Integrable Universal EquationsJul 13 1992Jul 23 1992Recent work on Euler hierarchies of field theory Lagrangians iteratively constructed {}from their successive equations of motion is briefly reviewed. On the one hand, a certain triality structure is described, relating arbitrary field theories, {\it classical\ts} ... More
The AdS3 boundary energy momentum tensor, exact in the string length over the curvature radiusJun 22 2010We first clarify the relation between boundary perturbations of AdS3 in general relativity, and exactly marginal worldsheet vertex operators in AdS3 string theory with Neveu-Schwarz Neveu-Schwarz flux. The latter correspond to solutions of the higher ... More
The non-compact elliptic genus: mock or modularApr 21 2010Jun 17 2010We analyze various perspectives on the elliptic genus of non-compact supersymmetric coset conformal field theories with central charge larger than three. We calculate the holomorphic part of the elliptic genus via a free field description of the model, ... More
Non-accessible critical points of Cremer polynomialsFeb 12 1995It is shown that a polynomial with a Cremer periodic point has a non-accessible critical point in its Julia set provided that the Cremer periodic point is approximated by small cycles.