3,892 research outputs found
Coding Schemes for Distributed Storage Systems: Implementation and Improvements
Distributed data storage systems are used to store data reliably over a distributed collection of storage locations, called peers. Coding schemes are used to store a portion of the data in the peers ensuring the complete retrieval of data, during peer failures. This has applications in various areas like Wireless Networks, Sensor Networks etc. In this framework we consider a large file to be stored in a distributed manner over few peers of limited capacity. Each peer stores a portion of the coded data, without the knowledge of the contents of other peers. Random Coding is one of the coding schemes used for this. In [1] coding coefficients are chosen randomly from a finite field to encode the data. The encoding is basically a linear combination of file pieces (pieces are elements of finite fields). The data downloader downloads these coded data from several peers and decodes to get the original data. The decoding is basically solving a system of linear equations over a finite field, which is the most time consuming step in the whole process. We give a simple C++ implementation of the schemes in [1] and plot the results. We are trying to find a scheme where coding vectors can be chosen such that the decoding complexity is reduced significantly. Also in a dynamic setting where nodes enter and leave system intermittently, are discussed
Deep Inelastic Scattering in Improved Lattice QCD. I. The first moment of structure functions
We present the complete 1-loop perturbative computation of the
renormalization constants and mixing coefficients of the operators that measure
the first moment of deep inelastic scattering structure functions, employing
the nearest neighbor improved lattice QCD action. The interest of using this
action in Monte Carlo simulations lies in the fact that all terms which in the
continuum limit are effectively of order ( being the lattice spacing)
have been proven to be absent from on-shell hadronic lattice matrix elements.
Because of the complexity of the calculations, we have checked the analytical
expression of all Feynman diagrams using Schoonschip. To this end we have
developed a suitable code designed to automatically carry out all the necessary
lattice algebraic manipulations, starting from the elementary building blocks
of each diagram.
We have found discrepancies with some of the published numbers, but we are in
agreement with the known results on the energy-momentum tensor.Comment: 59 pages, plain LaTeX + Feynman.tex (complete postscript file
available upon request to [email protected]), Preprint Roma1
978-93 and ROM2F 93/38 (some numerical mistakes have been corrected in Sects.
2.2 and 8
Perfect Lattice Actions for Staggered Fermions
We construct a perfect lattice action for staggered fermions by blocking from
the continuum. The locality, spectrum and pressure of such perfect staggered
fermions are discussed. We also derive a consistent fixed point action for free
gauge fields and discuss its locality as well as the resulting static
quark-antiquark potential. This provides a basis for the construction of
(classically) perfect lattice actions for QCD using staggered fermions.Comment: 30 pages, LaTex, 10 figure
The Schwinger Model with Perfect Staggered Fermions
We construct and test a quasi-perfect lattice action for staggered fermions.
The construction starts from free fermions, where we suggest a new blocking
scheme, which leads to excellent locality of the perfect action. An adequate
truncation preserves a high quality of the free action. An Abelian gauge field
is inserted in d=2 by effectively tuning the couplings to a few short-ranged
lattice paths, based on the behavior of topological zero modes. We simulate the
Schwinger model with this action, applying a new variant of Hybrid Monte Carlo,
which damps the computational overhead due to the non-standard couplings. We
obtain a tiny ``pion'' mass down to very small \beta, while the ``\eta'' mass
follows very closely the prediction of asymptotic scaling. The observation that
even short-ranged quasi-perfect actions can yield strong improvement is most
relevant in view of QCD.Comment: 30 pages, 16 figures. Following the referee's suggestions, we have
incorporated the material of hep-lat/9803018 in this comprehensive pape
Phase-resolved spectral analysis of 4U 1901+03 during its outburst
The high mass X-ray binary 4U 1901+03 was reported to have the pulse profile
evolving with the X-ray luminosity and energy during its outburst in
February-July 2003: the pulse peak changed from double to single along with the
decreasing luminosity. We have carried out a detailed analysis on the
contemporary phase-resolved energy spectrum of 4U 1901+03 as observed by Rossi
X-ray Timing Explorer (RXTE). We find that, both the continuum and the pulse
spectra are phase dependent. The optical depth derived from the pulse spectrum
is in general larger than that from the continuum. Fe Ka emission line is only
detected in the spectrum of the continuum and is missing in the pulse spectrum.
This suggests an origin of Fe emission from the accretion disk but not the
surface of the neutron star.Comment: 18 pages, 8 figures, accepted by Ap
Australian community pharmacy services: a survey of what people with chronic conditions and their carers use versus what they consider important
Objective To explore the purpose/s for which people with chronic conditions and their carers use Australian community pharmacies, and compare this to what pharmacy services they consider important, from the perspectives of both consumers and pharmacists. Design An exploratory study involving a survey, which asked participants to indicate the pharmacy services they had ever used, and rate the importance of 22 pharmacy services to them, or the person they care for, or for their consumers if a pharmacist. Setting Four regions of Australia: Logan-Beaudesert and Mt Isa/North West region, Queensland, Northern Rivers, New South Wales, and the Greater Perth area, Western Australia. Participants Surveys were undertaken with 602 consumers and 91 community pharmacists. Results Community pharmacy is predominantly used to obtain advice about medication and whether a doctor's visit is necessary, as well as for monitoring and screening services. Pharmacy services that were patient centric were important, such as individualised medication advice and respectful care, as well as tools or procedures to facilitate streamlined medication access. Less important services included adult vaccinations and health and wellness programmes. Carers identified services that assisted them with their specific role/s to be important. Overall, community pharmacists had a good understanding of the services that were important to people with chronic conditions and their carers. Conclusions People with chronic conditions and their carers not only care about what services are delivered, but how they are delivered; they sought services that generally improved their access to medication and information, but in a way that was patient centred. Ultimately, pharmacists understood the importance of patient-centred care for people with chronic conditions and their carers, perhaps indicating a greater acceptance of integrating patient-centred care into their everyday practice
Baryon masses at O(a^2) in chiral perturbation theory
The chiral Lagrangian for the Symanzik action through O(a^2) for baryons is
obtained. We consider two flavor unquenched and partially quenched lattice
theories, allowing for mixed actions in the latter. As an application, we
calculate masses to O(a^2) for the nucleons and deltas, and investigate the
corrections due to the violation of O(4) rotational invariance. These results
are contrasted with those in the meson sector for lattice simulations using
mixed and unmixed actions of Wilson and Ginsparg-Wilson quarks.Comment: 27 pages, 2 figures, revise
Resolved Sideband Cooling of a Micromechanical Oscillator
Micro- and nanoscale opto-mechanical systems provide radiation pressure
coupling of optical and mechanical degree of freedom and are actively pursued
for their ability to explore quantum mechanical phenomena of macroscopic
objects. Many of these investigations require preparation of the mechanical
system in or close to its quantum ground state. Remarkable progress in ground
state cooling has been achieved for trapped ions and atoms confined in optical
lattices. Imperative to this progress has been the technique of resolved
sideband cooling, which allows overcoming the inherent temperature limit of
Doppler cooling and necessitates a harmonic trapping frequency which exceeds
the atomic species' transition rate. The recent advent of cavity back-action
cooling of mechanical oscillators by radiation pressure has followed a similar
path with Doppler-type cooling being demonstrated, but lacking inherently the
ability to attain ground state cooling as recently predicted. Here we
demonstrate for the first time resolved sideband cooling of a mechanical
oscillator. By pumping the first lower sideband of an optical microcavity,
whose decay rate is more than twenty times smaller than the eigen-frequency of
the associated mechanical oscillator, cooling rates above 1.5 MHz are attained.
Direct spectroscopy of the motional sidebands reveals 40-fold suppression of
motional increasing processes, which could enable reaching phonon occupancies
well below unity (<0.03). Elemental demonstration of resolved sideband cooling
as reported here should find widespread use in opto-mechanical cooling
experiments. Apart from ground state cooling, this regime allows realization of
motion measurement with an accuracy exceeding the standard quantum limit.Comment: 13 pages, 5 figure
Minimization of phonon-tunneling dissipation in mechanical resonators
Micro- and nanoscale mechanical resonators have recently emerged as
ubiquitous devices for use in advanced technological applications, for example
in mobile communications and inertial sensors, and as novel tools for
fundamental scientific endeavors. Their performance is in many cases limited by
the deleterious effects of mechanical damping. Here, we report a significant
advancement towards understanding and controlling support-induced losses in
generic mechanical resonators. We begin by introducing an efficient numerical
solver, based on the "phonon-tunneling" approach, capable of predicting the
design-limited damping of high-quality mechanical resonators. Further, through
careful device engineering, we isolate support-induced losses and perform the
first rigorous experimental test of the strong geometric dependence of this
loss mechanism. Our results are in excellent agreement with theory,
demonstrating the predictive power of our approach. In combination with recent
progress on complementary dissipation mechanisms, our phonon-tunneling solver
represents a major step towards accurate prediction of the mechanical quality
factor.Comment: 12 pages, 4 figure
Patterns of variability in Be/X-ray pulsars during giant outbursts
The discovery of source states in the X-ray emission of black-hole binaries
and neutron-star low-mass X-ray binaries constituted a major step forward in
the understanding of the physics of accretion onto compact objects. While there
are numerous studies on the correlated timing and spectral variability of these
systems, very little work has been done on high-mass X-ray binaries, the third
major type of X-ray binaries. The main goal of this work is to investigate
whether Be accreting X-ray pulsars display source states and characterise those
states through their spectral and timing properties. We have made a systematic
study of the power spectra, energy spectra and X-ray hardness-intensity
diagrams of nine Be/X-ray pulsars. The evolution of the timing and spectral
parameters were monitored through changes over two orders of magnitude in
luminosity. We find that Be/X-ray pulsars trace two different branches in the
hardness-intensity diagram: the horizontal branch corresponds to a
low-intensity state of the source and it is characterised by fast colour and
spectral changes and high X-ray variability. The diagonal branch is a
high-intensity state that emerges when the X-ray luminosity exceeds a critical
limit. The photon index anticorrelates with X-ray flux in the horizontal branch
but correlates with it in the diagonal branch. The correlation between QPO
frequency and X-ray flux reported in some pulsars is also observed if the peak
frequency of the broad-band noise that accounts for the aperiodic variability
is used. The two branches may reflect two different accretion modes, depending
on whether the luminosity of the source is above or below a critical value.
This critical luminosity is mainly determined by the magnetic field strength,
hence it differs for different sources.Comment: Complete missing words in title. Proof corrections adde
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