3,392 research outputs found
Throughput Maximization in Multiprocessor Speed-Scaling
We are given a set of jobs that have to be executed on a set of
speed-scalable machines that can vary their speeds dynamically using the energy
model introduced in [Yao et al., FOCS'95]. Every job is characterized by
its release date , its deadline , its processing volume if
is executed on machine and its weight . We are also given a budget
of energy and our objective is to maximize the weighted throughput, i.e.
the total weight of jobs that are completed between their respective release
dates and deadlines. We propose a polynomial-time approximation algorithm where
the preemption of the jobs is allowed but not their migration. Our algorithm
uses a primal-dual approach on a linearized version of a convex program with
linear constraints. Furthermore, we present two optimal algorithms for the
non-preemptive case where the number of machines is bounded by a fixed
constant. More specifically, we consider: {\em (a)} the case of identical
processing volumes, i.e. for every and , for which we
present a polynomial-time algorithm for the unweighted version, which becomes a
pseudopolynomial-time algorithm for the weighted throughput version, and {\em
(b)} the case of agreeable instances, i.e. for which if and only
if , for which we present a pseudopolynomial-time algorithm. Both
algorithms are based on a discretization of the problem and the use of dynamic
programming
The free energy in a class of quantum spin systems and interchange processes
We study a class of quantum spin systems in the mean-field setting of the
complete graph. For spin the model is the Heisenberg ferromagnet,
for general spin it has a probabilistic representation
as a cycle-weighted interchange process. We determine the free energy and the
critical temperature (recovering results by T\'oth and by Penrose when
). The critical temperature is shown to coincide (as a function of
) with that of the state classical Potts model, and the phase
transition is discontinuous when .Comment: 22 page
First-Principle Description of Correlation Effects in Layered Materials
We present a first-principles description of anisotropic materials
characterized by having both weak (dispersion-like) and strong covalent bonds,
based on the Adiabatic--Connection Fluctuation--Dissipation Theorem within
Density Functional Theory. For hexagonal boron nitride the in-plane and out of
plane bonding as well as vibrational dynamics are well described both at
equilibrium and when the layers are pulled apart. Also bonding in covalent and
ionic solids is described. The formalism allows to ping-down the deficiencies
of common exchange-correlation functionals and provides insight towards the
inclusion of dispersion interactions into the correlation functional.Comment: Accepted for publication in Physical Review Letter
Obtaining efficient collisional engines via velocity dependent drivings
Brownian particles interacting sequentially with distinct temperatures and
driving forces at each stroke have been tackled as a reliable alternative for
the construction of engine setups. However they can behave very inefficiently
depending on the driving used for the worksource and/or when temperatures of
each stage are very different from each other. Inspired by some models for
molecular motors and recent experimental studies, a coupling between driving
and velocities is introduced as an alternative ingredient for enhancing the
system performance. Here, the role of this new ingredient for levering the
engine performance is detailed investigated from stochastic thermodynamics.
Exact expressions for quantities and distinct maximization routes have been
obtained and investigated. The search of an optimal coupling provides a
substantial increase of engine performance (mainly efficiency), even for large
. A simple and general argument for the optimal coupling can be
estimated, irrespective the driving and other model details.Comment: 10 pages, 8 figures, comments are welcom
Critical phase in non-conserving zero-range processes and equilibrium networks
Zero-range processes, in which particles hop between sites on a lattice, are
closely related to equilibrium networks, in which rewiring of links take place.
Both systems exhibit a condensation transition for appropriate choices of the
dynamical rules. The transition results in a macroscopically occupied site for
zero-range processes and a macroscopically connected node for networks.
Criticality, characterized by a scale-free distribution, is obtained only at
the transition point. This is in contrast with the widespread scale-free
real-life networks. Here we propose a generalization of these models whereby
criticality is obtained throughout an entire phase, and the scale-free
distribution does not depend on any fine-tuned parameter.Comment: 4 pages, 4 figure
Improving the Price of Anarchy for Selfish Routing via Coordination Mechanisms
We reconsider the well-studied Selfish Routing game with affine latency
functions. The Price of Anarchy for this class of games takes maximum value
4/3; this maximum is attained already for a simple network of two parallel
links, known as Pigou's network. We improve upon the value 4/3 by means of
Coordination Mechanisms.
We increase the latency functions of the edges in the network, i.e., if
is the latency function of an edge , we replace it by
with for all . Then an
adversary fixes a demand rate as input. The engineered Price of Anarchy of the
mechanism is defined as the worst-case ratio of the Nash social cost in the
modified network over the optimal social cost in the original network.
Formally, if \CM(r) denotes the cost of the worst Nash flow in the modified
network for rate and \Copt(r) denotes the cost of the optimal flow in the
original network for the same rate then [\ePoA = \max_{r \ge 0}
\frac{\CM(r)}{\Copt(r)}.]
We first exhibit a simple coordination mechanism that achieves for any
network of parallel links an engineered Price of Anarchy strictly less than
4/3. For the case of two parallel links our basic mechanism gives 5/4 = 1.25.
Then, for the case of two parallel links, we describe an optimal mechanism; its
engineered Price of Anarchy lies between 1.191 and 1.192.Comment: 17 pages, 2 figures, preliminary version appeared at ESA 201
Electron dynamics in intentionally disordered semiconductor superlattices
We study the dynamical behavior of disordered quantum-well-based
semiconductor superlattices where the disorder is intentional and short-range
correlated. We show that, whereas the transmission time of a particle grows
exponentially with the number of wells in an usual disordered superlattice for
any value of the incident particle energy, for specific values of the incident
energy this time increases linearly when correlated disorder is included. As
expected, those values of the energy coincide with a narrow subband of extended
states predicted by the static calculations of Dom\'{\i}nguez-Adame {\em et
al.} [Phys. Rev. B {\bf 51}, 14 ,359 (1994)]; such states are seen in our
dynamical results to exhibit a ballistic regime, very close to the WKB
approximation of a perfect superlattice. Fourier transform of the output signal
for an incident Gaussian wave packet reveals a dramatic filtering of the
original signal, which makes us confident that devices based on this property
may be designed and used for nanotechnological applications. This is more so in
view of the possibility of controllingthe outp ut band using a dc electric
field, which we also discuss. In the conclusion we summarize our results and
present an outlook for future developments arising from this work.Comment: 10 pagex, RevTex, 13 Postscript figures. Physical Review B (in press
A note on the existence of standard splittings for conformally stationary spacetimes
Let be a spacetime which admits a complete timelike conformal Killing
vector field . We prove that splits globally as a standard
conformastationary spacetime with respect to if and only if is
distinguishing (and, thus causally continuous). Causal but non-distinguishing
spacetimes with complete stationary vector fields are also exhibited. For the
proof, the recently solved "folk problems" on smoothability of time functions
(moreover, the existence of a {\em temporal} function) are used.Comment: Metadata updated, 6 page
Acceptability and facilitators of and barriers to point-of-care HIV testing in a homeless-focused service in Gloucestershire: a qualitative evaluation
Objectives: Late HIV diagnosis increases the risks of onward transmission, morbidity and mortality. Rapid point-of-care testing (POCT) reaches people who have never been tested and people living with HIV who are undiagnosed. This study explored the acceptability and feasibility of HIV POCT from the perspectives of service providers and users. //
Methods: A pilot study introduced HIV POCT to one service in Gloucestershire, England. Eleven semi-structured interviews with service users and a focus group with three service providers were conducted. The Theoretical Framework of Acceptability and the Theoretical Domains Framework were used to design the topic guide and analysis. //
Results: Acceptability of HIV POCT was high. Seven facilitators were identified (e.g. understanding the test purpose and process), alongside two potential barriers, one relevant to service providers and users (anxiety) and the other to service users (stigma). // Conclusions: To maximize the benefits of implementation of HIV POCT, health care providers require appropriate training and supervision to offer and administer POCT
Effective field theory and dispersion law of the phonons of a non-relativistic superfluid
We study the recently proposed effective field theory for the phonon of an
arbitrary non-relativistic superfluid. After computing the one-loop phonon
self-energy, we obtain the low temperature T contributions to the phonon
dispersion law at low momentum, and see that the real part of those can be
parametrized as a thermal correction to the phonon velocity. Because the
phonons are the quanta of the sound waves, at low momentum their velocity
should agree with the speed of sound. We find that our results match at order
T^4ln(T) with those predicted by Andreev and Khalatnikov for the speed of
sound, derived from the superfluid hydrodynamical equations and the phonon
kinetic theory. We get also higher order corrections of order T^4, which are
not reproduced pushing naively the kinetic theory computation. Finally, as an
application, we consider the cold Fermi gas in the unitarity limit, and find a
universal expression for the low T relative correction to the speed of sound
for these systems.Comment: 14 pages, 2 figures. References adde
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