279 research outputs found
Collective excitation frequencies of Bosons in a parabolic potential with interparticle harmonic interactions
The fact that the ground-state first-order density matrix for Bosons in a
parabolic potential with interparticle harmonic interactions is known in exact
form is here exploited to study collective excitations in the weak-coupling
regime. Oscillations about the ground-state density are treated analytically by
a linearized equation of motion which includes a kinetic energy contribution.
We show that the dipole mode has the frequency of the bare trap, in accord with
the Kohn theorem, and derive explicit expressions for the frequencies of the
higher-multipole modes in terms of a frequency renormalized by the
interactions.Comment: 6 pages, no figures, accepted for publication on Physics Letters
Convergence of the all-time supremum of a L\'evy process in the heavy-traffic regime
In this paper we derive a technique of obtaining limit theorems for suprema
of L\'evy processes from their random walk counterparts. For each , let
be a sequence of independent and identically distributed
random variables and be a L\'evy processes such that
, and as . Let .
Then, under some mild assumptions, , for some random variable and some function
. We utilize this result to present a number of limit theorems
for suprema of L\'evy processes in the heavy-traffic regime
A comparison of limited-stretch models of rubber elasticity
In this paper we describe various limited-stretch models of non-linear rubber elasticity, each dependent on only the first invariant of the left Cauchy-Green strain tensor and having only two independent material constants. The models are described as limited-stretch, or restricted elastic, because the strain energy and stress response become infinite at a finite value of the first invariant. These models describe well the limited stretch of the polymer chains of which rubber is composed. We discuss Gent's model which is the simplest limited-stretch model and agrees well with experiment. Various statistical models are then described: the one-chain, three-chain, four-chain and Arruda-Boyce eight-chain models, all of which involve the inverse Langevin function. A numerical comparison between the three-chain and eight-chain models is provided. Next, we compare various models which involve approximations to the inverse Langevin function with the exact inverse Langevin function of the eight-chain model. A new approximate model is proposed that is as simple as Cohen's original model but significantly more accurate. We show that effectively the eight-chain model may be regarded as a linear combination of the neo-Hookean and Gent models. Treloar's model is shown to have about half the percentage error of our new model but it is much more complicated. For completeness a modified Treloar model is introduced but this is only slightly more accurate than Treloar's original model. For the deformations of uniaxial tension, biaxial tension, pure shear and simple shear we compare the accuracy of these models, and that of Puso, with the eight-chain model by means of graphs and a table. Our approximations compare extremely well with models frequently used and described in the literature, having the smallest mean percentage error over most of the range of the argument
Programming Robots With Events
International audienceWe introduce how to use event-based style to program robots through the INI programming language. INI features both built-in and user-defined events, a mechanism to handle various kinds of changes happening in the environment. Event handlers run in parallel either synchronously or asynchronously, and events can be reconfigured at runtime to modify their behavior when needed. We apply INI to the humanoid robot called Nao, for which we develop an object tracking program
Hardware and software status of QCDOC
QCDOC is a massively parallel supercomputer whose processing nodes are based
on an application-specific integrated circuit (ASIC). This ASIC was
custom-designed so that crucial lattice QCD kernels achieve an overall
sustained performance of 50% on machines with several 10,000 nodes. This strong
scalability, together with low power consumption and a price/performance ratio
of $1 per sustained MFlops, enable QCDOC to attack the most demanding lattice
QCD problems. The first ASICs became available in June of 2003, and the testing
performed so far has shown all systems functioning according to specification.
We review the hardware and software status of QCDOC and present performance
figures obtained in real hardware as well as in simulation.Comment: Lattice2003(machine), 6 pages, 5 figure
Understanding person acquisition using an interactive activation and competition network
Face perception is one of the most developed visual skills that humans display, and recent work has attempted to examine the mechanisms involved in face perception through noting how neural networks achieve the same performance. The purpose of the present paper is to extend this approach to look not just at human face recognition, but also at human face acquisition. Experiment 1 presents empirical data to describe the acquisition over time of appropriate representations for newly encountered faces. These results are compared with those of Simulation 1, in which a modified IAC network capable of modelling the acquisition process is generated. Experiment 2 and Simulation 2 explore the mechanisms of learning further, and it is demonstrated that the acquisition of a set of associated new facts is easier than the acquisition of individual facts in isolation of one another. This is explained in terms of the advantage gained from additional inputs and mutual reinforcement of developing links within an interactive neural network system. <br/
Influence of thermal conductivity on the dynamic response of magnetocaloric materials
We compare the magnetocaloric effect of samples prepared with different thermal conductivities to investigate the potential of composite materials. By applying the magnetic field under operating conditions we test the material's response and compare this to heat transfer simulations in order to check the reliability of the adiabatic temperature change probe used. As a result of this study we highlight how the material's thermal conductivity influences Full-size image (<1 K), the time constant of temperature change. This parameter ultimately limits the maximum frequency of a refrigerant cycle and offers fundamental information about the correlation between thermal conductivity and the magnetocaloric effect
Lubricating Bacteria Model for Branching growth of Bacterial Colonies
Various bacterial strains (e.g. strains belonging to the genera Bacillus,
Paenibacillus, Serratia and Salmonella) exhibit colonial branching patterns
during growth on poor semi-solid substrates. These patterns reflect the
bacterial cooperative self-organization. Central part of the cooperation is the
collective formation of lubricant on top of the agar which enables the bacteria
to swim. Hence it provides the colony means to advance towards the food. One
method of modeling the colonial development is via coupled reaction-diffusion
equations which describe the time evolution of the bacterial density and the
concentrations of the relevant chemical fields. This idea has been pursued by a
number of groups. Here we present an additional model which specifically
includes an evolution equation for the lubricant excreted by the bacteria. We
show that when the diffusion of the fluid is governed by nonlinear diffusion
coefficient branching patterns evolves. We study the effect of the rates of
emission and decomposition of the lubricant fluid on the observed patterns. The
results are compared with experimental observations. We also include fields of
chemotactic agents and food chemotaxis and conclude that these features are
needed in order to explain the observations.Comment: 1 latex file, 16 jpeg files, submitted to Phys. Rev.
The Ties that Double Bind Us: Career, Emotion and Narrative Coping in Difficult Working Relationships
This article examines through an autoethnographic account how career aspirations and constraints may lead individuals to endure emotionally aversive situations. It presents evidence that individuals in such situations engage in emotion‐focused coping through narrative, illustrated by the author’s autoethnographic narrative of a difficult working relationship which developed into a double bind situation. The paper suggests that narrative coping in response to a double bind can actually serve to reify and prolong such situations. The paper concludes that autoethnographic research does not lend itself to simple organisational solutions. Possible avenues for further research are outlined and discussed
Queues with Lévy input and hysteretic control
We consider a (doubly) reflected Lévy process where the Lévy exponent is controlled by a hysteretic policy consisting of two stages. In each stage there is typically a different service speed, drift parameter, or arrival rate. We determine the steady-state performance, both for systems with finite and infinite capacity. Thereby, we unify and extend many existing results in the literature, focusing on the special cases of M/G/1 queues and Brownian motion. © The Author(s) 2009
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