4,810 research outputs found
Parallel Batch-Dynamic Graph Connectivity
In this paper, we study batch parallel algorithms for the dynamic
connectivity problem, a fundamental problem that has received considerable
attention in the sequential setting. The most well known sequential algorithm
for dynamic connectivity is the elegant level-set algorithm of Holm, de
Lichtenberg and Thorup (HDT), which achieves amortized time per
edge insertion or deletion, and time per query. We
design a parallel batch-dynamic connectivity algorithm that is work-efficient
with respect to the HDT algorithm for small batch sizes, and is asymptotically
faster when the average batch size is sufficiently large. Given a sequence of
batched updates, where is the average batch size of all deletions, our
algorithm achieves expected amortized work per
edge insertion and deletion and depth w.h.p. Our algorithm
answers a batch of connectivity queries in expected
work and depth w.h.p. To the best of our knowledge, our algorithm
is the first parallel batch-dynamic algorithm for connectivity.Comment: This is the full version of the paper appearing in the ACM Symposium
on Parallelism in Algorithms and Architectures (SPAA), 201
Quark-Hadron Phase Transitions in Viscous Early Universe
Based on hot big bang theory, the cosmological matter is conjectured to
undergo QCD phase transition(s) to hadrons, when the universe was about s old. In the present work, we study the quark-hadron phase transition, by
taking into account the effect of the bulk viscosity. We analyze the evolution
of the quantities relevant for the physical description of the early universe,
namely, the energy density , temperature , Hubble parameter and
scale factor before, during and after the phase transition. To study the
cosmological dynamics and the time evolution we use both analytical and
numerical methods. By assuming that the phase transition may be described by an
effective nucleation theory (prompt {\it first-order} phase transition), we
also consider the case where the universe evolved through a mixed phase with a
small initial supercooling and monotonically growing hadronic bubbles. The
numerical estimation of the cosmological parameters, and for instance,
makes it clear that the time evolution varies from phase to phase. As the QCD
era turns to be fairly accessible in the high-energy experiments and the
lattice QCD simulations, the QCD equation of state is very well defined. In
light of this, we introduce a systematic study of the {\it cross-over}
quark-hadron phase transition and an estimation for the time evolution of
Hubble parameter.Comment: 27 pages, 17 figures, revtex style (To appear in Phys. Rev. D). arXiv
admin note: text overlap with arXiv:gr-qc/040404
Distribution of occupation numbers in finite Fermi-systems and role of interaction in chaos and thermalization
New method is developed for calculation of single-particle occupation numbers
in finite Fermi systems of interacting particles. It is more accurate than the
canonical distribution method and gives the Fermi-Dirac distribution in the
limit of large number of particles. It is shown that statistical effects of the
interaction are absorbed by an increase of the effective temperature. Criteria
for quantum chaos and statistical equilibrium are considered. All results are
confirmed by numerical experiments in the two-body random interaction model.Comment: 4 pages, Latex, 4 figures in the form of PS-file
Electron-phonon coupling and longitudinal mechanical-mode cooling in a metallic nanowire
We investigate electron-phonon coupling in a narrow suspended metallic wire,
in which the phonon modes are restricted to one dimension but the electrons
behave three-dimensionally. Explicit theoretical results related to the known
bulk properties are derived. We find out that longitudinal vibration modes can
be cooled by electronic tunnel refrigeration far below the bath temperature
provided the mechanical quality factors of the modes are sufficiently high. The
obtained results apply to feasible experimental configurations.Comment: 4+ pages, 3 figure
The effect of alternative permutation testing strategies on the performance of multifactor dimensionality reduction
<p>Abstract</p> <p>Background</p> <p>Multifactor Dimensionality Reduction (MDR) is a novel method developed to detect gene-gene interactions in case-control association analysis by exhaustively searching multi-locus combinations. While the end-goal of analysis is hypothesis generation, significance testing is employed to indicate statistical interest in a resulting model. Because the underlying distribution for the null hypothesis of no association is unknown, non-parametric permutation testing is used. Lately, there has been more emphasis on selecting all statistically significant models at the end of MDR analysis in order to avoid missing a true signal. This approach opens up questions about the permutation testing procedure. Traditionally omnibus permutation testing is used, where one permutation distribution is generated for all models. An alternative is <it>n</it>-locus permutation testing, where a separate distribution is created for each <it>n</it>-level of interaction tested.</p> <p>Findings</p> <p>In this study, we show that the false positive rate for the MDR method is at or below a selected alpha level, and demonstrate the conservative nature of omnibus testing. We compare the power and false positive rates of both permutation approaches and find omnibus permutation testing optimal for preserving power while protecting against false positives.</p> <p>Conclusion</p> <p>Omnibus permutation testing should be used with the MDR method.</p
BLITZEN: A highly integrated massively parallel machine
The architecture and VLSI design of a new massively parallel processing array chip are described. The BLITZEN processing element array chip, which contains 1.1 million transistors, serves as the basis for a highly integrated, miniaturized, high-performance, massively parallel machine that is currently under development. Each processing element has 1K bits of static RAM and performs bit-serial processing with functional elements for arithmetic, logic, and shifting
Quasi-Particle Degrees of Freedom versus the Perfect Fluid as Descriptors of the Quark-Gluon Plasma
The hot nuclear matter created at the Relativistic Heavy Ion Collider (RHIC)
has been characterized by near-perfect fluid behavior. We demonstrate that this
stands in contradiction to the identification of QCD quasi-particles with the
thermodynamic degrees of freedom in the early (fluid) stage of heavy ion
collisions. The empirical observation of constituent quark ``'' scaling of
elliptic flow is juxtaposed with the lack of such scaling behavior in
hydrodynamic fluid calculations followed by Cooper-Frye freeze-out to hadrons.
A ``quasi-particle transport'' time stage after viscous effects break down the
hydrodynamic fluid stage, but prior to hadronization, is proposed to reconcile
these apparent contradictions. However, without a detailed understanding of the
transitions between these stages, the ``'' scaling is not a necessary
consequence of this prescription. Also, if the duration of this stage is too
short, it may not support well defined quasi-particles. By comparing and
contrasting the coalescence of quarks into hadrons with the similar process of
producing light nuclei from nucleons, it is shown that the observation of
``'' scaling in the final state does not necessarily imply that the
constituent degrees of freedom were the relevant ones in the initial state.Comment: 9 pages, 7 figures, Updated text and figure
Generalized Kinetic Theory of Electrons and Phonons: Models, Equilibrium, Stability
In the present paper our aim is to introduce some models for the
generalization of the kinetic theory of electrons and phonons (KTEP), as well
as to study equilibrium solutions and their stability for the generalized KTEP
(GKTEP) equations. We consider a couple of models, relevant to non standard
quantum statistics, which give rise to inverse power law decays of the
distribution function with respect to energy. In the case of electrons in a
phonon background, equilibrium and stability are investigated by means of
Lyapounov theory. Connections with thermodynamics are pointed out.Comment: 10 pages, 2 figures, (RevTeX4), to appear in Physica B (2003
Quantum mechanical sum rules for two model systems
Sum rules have played an important role in the development of many branches
of physics since the earliest days of quantum mechanics. We present examples of
one-dimensional quantum mechanical sum rules and apply them in two familiar
systems, the infinite well and the single delta-function potential. These cases
illustrate the different ways in which such sum rules can be realized, and the
varying mathematical techniques by which they can be confirmed. Using the same
methods, we also evaluate the second-order energy shifts arising from the
introduction of a constant external field, namely the Stark effect.Comment: 23 pages, no figures, to appear in Am. J. Phy
Radial Spin Helix in Two-Dimensional Electron Systems with Rashba Spin-Orbit Coupling
We suggest a long-lived spin polarization structure, a radial spin helix, and
study its relaxation dynamics. For this purpose, starting with a simple and
physically clear consideration of spin transport, we derive a system of
equations for spin polarization density and find its general solution in the
axially symmetric case. It is demonstrated that the radial spin helix of a
certain period relaxes slower than homogeneous spin polarization and plain spin
helix. Importantly, the spin polarization at the center of the radial spin
helix stays almost unchanged at short times. At longer times, when the initial
non-exponential relaxation region ends, the relaxation of the radial spin helix
occurs with the same time constant as that describing the relaxation of the
plain spin helix.Comment: 9 pages, 7 figure
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