6,632 research outputs found
Deformations of symplectic cohomology and exact Lagrangians in ALE spaces
We prove that the only exact Lagrangian submanifolds in an ALE space are
spheres. ALE spaces are the simply connected hyperkahler manifolds which at
infinity look like C^2/G for any finite subgroup G of SL(2,C). They can be
realized as the plumbing of copies of the cotangent bundle of a 2-sphere
according to ADE Dynkin diagrams. The proof relies on symplectic cohomology.Comment: 35 pages, 3 figures, minor changes and corrected typo
Principal manifolds and graphs in practice: from molecular biology to dynamical systems
We present several applications of non-linear data modeling, using principal
manifolds and principal graphs constructed using the metaphor of elasticity
(elastic principal graph approach). These approaches are generalizations of the
Kohonen's self-organizing maps, a class of artificial neural networks. On
several examples we show advantages of using non-linear objects for data
approximation in comparison to the linear ones. We propose four numerical
criteria for comparing linear and non-linear mappings of datasets into the
spaces of lower dimension. The examples are taken from comparative political
science, from analysis of high-throughput data in molecular biology, from
analysis of dynamical systems.Comment: 12 pages, 9 figure
A controlled experiment for the empirical evaluation of safety analysis techniques for safety-critical software
Context: Today's safety critical systems are increasingly reliant on
software. Software becomes responsible for most of the critical functions of
systems. Many different safety analysis techniques have been developed to
identify hazards of systems. FTA and FMEA are most commonly used by safety
analysts. Recently, STPA has been proposed with the goal to better cope with
complex systems including software. Objective: This research aimed at comparing
quantitatively these three safety analysis techniques with regard to their
effectiveness, applicability, understandability, ease of use and efficiency in
identifying software safety requirements at the system level. Method: We
conducted a controlled experiment with 21 master and bachelor students applying
these three techniques to three safety-critical systems: train door control,
anti-lock braking and traffic collision and avoidance. Results: The results
showed that there is no statistically significant difference between these
techniques in terms of applicability, understandability and ease of use, but a
significant difference in terms of effectiveness and efficiency is obtained.
Conclusion: We conclude that STPA seems to be an effective method to identify
software safety requirements at the system level. In particular, STPA addresses
more different software safety requirements than the traditional techniques FTA
and FMEA, but STPA needs more time to carry out by safety analysts with little
or no prior experience.Comment: 10 pages, 1 figure in Proceedings of the 19th International
Conference on Evaluation and Assessment in Software Engineering (EASE '15).
ACM, 201
Characterization of Sol-Gel-Derived Cobalt Oxide Xerogels as Electrochemical Capacitors
Very fine cobalt oxide xerogel powders were prepared using a unique solution chemistry associated with the sol-gel process. The effect of thermal treatment on the surface area, pore volume, crystallinity, particle structure, and corresponding electrochemical properties of the resulting xerogels was investigated and found to have significant effects on all of these properties. The xerogel remained amorphous as Co(OH)2 up to 160°C, and exhibited maxima in both the surface area and pore volume at this temperature. With an increase in the temperature above 200°C, both the surface area and pore volume decreased sharply, because the amorphous Co(OH)2 decomposed to form CoO that was subsequently oxidized to form crystalline Co3O4. In addition, the changes in the surface area, pore volume, crystallinity, and particle structure all had significant but coupled effects on the electrochemical properties of the xerogels. A maximum capacitance of 291 F/g was obtained for an electrode prepared with the CoOx xerogel calcined at 150°C, which was consistent with the maxima exhibited in both the surface area and pore volume; this capacitance was attributed solely to a surface redox mechanism. The cycle life of this electrode was also very stable for many thousands of cycles
LMC X-1: A New Spectral Analysis of the O-star in the binary and surrounding nebula
We provide new observations of the LMC X-1 O star and its extended nebula
structure using spectroscopic data from VLT/UVES as well as H imaging
from the Wide Field Imager on the Max Planck Gesellschaft / European Southern
Observatory 2.2m telescope and ATCA imaging of the 2.1 GHz radio continuum.
This nebula is one of the few known to be energized by an X-ray binary. We use
a new spectrum extraction technique that is superior to other methods to obtain
both radial velocities and fluxes. This provides an updated spatial velocity of
km s for the O star. The slit encompasses both the
photo-ionized and shock-ionized regions of the nebula. The imaging shows a
clear arc-like structure reminiscent of a wind bow shock in between the
ionization cone and shock-ionized nebula. The observed structure can be fit
well by the parabolic shape of a wind bow shock. If an interpretation of a wind
bow shock system is valid, we investigate the N159-O1 star cluster as a
potential parent of the system, suggesting a progenitor mass of
M for the black hole. We further note that the radio emission could
be non-thermal emission from the wind bow shock, or synchrotron emission
associated with the jet inflated nebula. For both wind and jet-powered origins,
this would represent one of the first radio detections of such a structure.Comment: 7 Figures, 4 Table
Spin correlations and Dzyaloshinskii-Moriya interaction in CsCuCl
We report on electron spin resonance (ESR) studies of the spin relaxation in
CsCuCl. The main source of the ESR linewidth at temperatures K is attributed to the uniform Dzyaloshinskii-Moriya interaction. The
vector components of the Dzyaloshinskii-Moriya interaction are determined from
the angular dependence of the ESR spectra using a high-temperature
approximation. Both the angular and temperature dependence of the ESR linewidth
have been analyzed using a self-consistent quantum-mechanical approach. In
addition analytical expressions based on a quasi-classical picture for spin
fluctuations are derived, which show good agreement with the quantum-approach
for temperatures K. A small modulation of the
ESR linewidth observed in the -plane is attributed to the anisotropic
Zeeman interaction, which reflects the two magnetically nonequivalent Cu
positions
The details of decriminalization: Designing a non-criminal response to the possession of drugs for personal use
Internationally, policymakers are considering alternative, non-criminal responses to the possession of drugs for personal use, or ‘simple possession’. We show that ‘decriminalization’ is not a simple, unified model; rather, there are meaningful differences in policies and options available as part of a non-criminal response. Responses include various decriminalization, diversion, and depenalization approaches. However, what details need to be considered in developing these approaches? In this paper, we eschew these labels and present an overview of key design features of non-criminal responses to simple possession and consider some of the equity considerations of the choices available, including reform architecture (the objectives and legal framework); eligibility criteria (population-, place-, and drug-based criteria); and actions taken (deterrence, therapeutic, and enforcement strategies). This paper does not evaluate individual features or models, but instead offers a practical framework that can be used to deliberate on potential reform decisions
Graphene on Si(111)7x7
We demonstrate that it is possible to mechanically exfoliate graphene under
ultra high vacuum conditions on the atomically well defined surface of single
crystalline silicon. The flakes are several hundred nanometers in lateral size
and their optical contrast is very faint in agreement with calculated data.
Single layer graphene is investigated by Raman mapping. The G and 2D peaks are
shifted and narrowed compared to undoped graphene. With spatially resolved
Kelvin probe measurements we show that this is due to p-type doping with hole
densities of n_h \simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique
presented here should open up new possibilities to influence the properties of
graphene by introducing adsorbates in a controlled way.Comment: 8 pages, 7 figure
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A high-wavenumber boundary-element method for an acoustic scattering problem
In this paper we show stability and convergence for a novel Galerkin boundary element method approach to the impedance boundary value problem for the Helmholtz equation in a half-plane with piecewise constant boundary data. This problem models, for example, outdoor sound propagation over inhomogeneous flat terrain. To achieve a good approximation with a relatively low number of degrees of freedom we employ a graded mesh with smaller elements adjacent to discontinuities in impedance, and a special set of basis functions for the Galerkin method so that, on each element, the approximation space consists of polynomials (of degree ) multiplied by traces of plane waves on the boundary. In the case where the impedance is constant outside an interval , which only requires the discretization of , we show theoretically and experimentally that the error in computing the acoustic field on is , where is the number of degrees of freedom and is the wavenumber. This indicates that the proposed method is especially commendable for large intervals or a high wavenumber. In a final section we sketch how the same methodology extends to more general scattering problems
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