6,308 research outputs found
Is Our Model for Contention Resolution Wrong?
Randomized binary exponential backoff (BEB) is a popular algorithm for
coordinating access to a shared channel. With an operational history exceeding
four decades, BEB is currently an important component of several wireless
standards. Despite this track record, prior theoretical results indicate that
under bursty traffic (1) BEB yields poor makespan and (2) superior algorithms
are possible. To date, the degree to which these findings manifest in practice
has not been resolved.
To address this issue, we examine one of the strongest cases against BEB:
packets that simultaneously begin contending for the wireless channel. Using
Network Simulator 3, we compare against more recent algorithms that are
inspired by BEB, but whose makespan guarantees are superior. Surprisingly, we
discover that these newer algorithms significantly underperform. Through
further investigation, we identify as the culprit a flawed but common
abstraction regarding the cost of collisions. Our experimental results are
complemented by analytical arguments that the number of collisions -- and not
solely makespan -- is an important metric to optimize. We believe that these
findings have implications for the design of contention-resolution algorithms.Comment: Accepted to the 29th ACM Symposium on Parallelism in Algorithms and
Architectures (SPAA 2017
Parallel algorithm with spectral convergence for nonlinear integro-differential equations
We discuss a numerical algorithm for solving nonlinear integro-differential
equations, and illustrate our findings for the particular case of Volterra type
equations. The algorithm combines a perturbation approach meant to render a
linearized version of the problem and a spectral method where unknown functions
are expanded in terms of Chebyshev polynomials (El-gendi's method). This
approach is shown to be suitable for the calculation of two-point Green
functions required in next to leading order studies of time-dependent quantum
field theory.Comment: 15 pages, 9 figure
Unmet Needs in the Pathogenesis and Treatment of Systemic Lupus Erythematosus
Systemic lupus erythematosus (SLE) is an autoimmune rheumatic disease with a prevalence of approximately 1 in 1000. Over the last 30Â years, advances in treatment such as use of corticosteroids and immunosuppressants have improved life expectancy and quality of life for patients with lupus and the key unmet needs have therefore changed. With the reduced mortality from disease activity, development of cardiovascular disease (CVD) has become an increasingly important cause of death in patients with SLE. The increased CVD risk in these patients is partly, but not fully explained by standard risk factors, and abnormalities in the immune response to lipids may play a role. Invariant natural killer T cells, which are triggered specifically by lipid antigens, may protect against progression of subclinical atherosclerosis. However, currently our recommendation is that clinicians should focus on optimal management of standard CVD risk factors such as smoking, blood pressure and lipid levels. Fatigue is one of the most common and most limiting symptoms suffered by patients with SLE. The cause of fatigue is multifactorial and disease activity does not explain this symptom. Consequently, therapies directed towards reducing inflammation and disease activity do not reliably reduce fatigue and new approaches are needed. Currently, we recommend asking about sleep pattern, optimising pain relief and excluding other causes of fatigue such as anaemia and metabolic disturbances. For the subgroup of patients whose disease activity is not fully controlled by standard treatment regimes, a range of different biologic agents have been proposed and subjected to clinical trials. Many of these trials have given disappointing results, though belimumab, which targets B lymphocytes, did meet its primary endpoint. New biologics targeting B cells, T cells or cytokines (especially interferon) are still going through trials raising the hope that novel therapies for patients with refractory SLE may be available soon
Case study: Anaesthesia implications and considerations in a case of pemphigus vulgaris for orthopaedic bipolar prosthesis implant surgery
A 60-year-old patient suffering from pemphigus vulgaris for the past year was admitted to the emergency ward for fracture neck of femur. She also presented with lesions involving oral mucosa, back, inframammary and genital areas which were in partial remission. In hospital she was diagnosed with hypertension and was put on anti-hypertensives. Special attention was paid during positioning for surgery, administration of regional anaesthesia and placement of the intravenous line as well as monitoring devices. General anaesthesia was avoided in the presence of partially active oral lesions. Combined spinal-epidural anaesthesia was administered using bupivacaine-clonidine mixture. No haemodynamic complication was observed with 30 μg of clonidine intrathecally and no skin lesion occurred at the site of injections or Tegaderm application
Numerical Approximations Using Chebyshev Polynomial Expansions
We present numerical solutions for differential equations by expanding the
unknown function in terms of Chebyshev polynomials and solving a system of
linear equations directly for the values of the function at the extrema (or
zeros) of the Chebyshev polynomial of order N (El-gendi's method). The
solutions are exact at these points, apart from round-off computer errors and
the convergence of other numerical methods used in connection to solving the
linear system of equations. Applications to initial value problems in
time-dependent quantum field theory, and second order boundary value problems
in fluid dynamics are presented.Comment: minor wording changes, some typos have been eliminate
1+1 Dimensional Compactifications of String Theory
We argue that stable, maximally symmetric compactifications of string theory
to 1+1 dimensions are in conflict with holography. In particular, the finite
horizon entropies of the Rindler wedge in 1+1 dimensional Minkowski and anti de
Sitter space, and of the de Sitter horizon in any dimension, are inconsistent
with the symmetries of these spaces. The argument parallels one made recently
by the same authors, in which we demonstrated the incompatibility of the
finiteness of the entropy and the symmetries of de Sitter space in any
dimension. If the horizon entropy is either infinite or zero the conflict is
resolved.Comment: 11 pages, 2 figures v2: added discussion of AdS_2 and comment
Holographic Construction of Excited CFT States
We present a systematic construction of bulk solutions that are dual to CFT
excited states. The bulk solution is constructed perturbatively in bulk fields.
The linearised solution is universal and depends only on the conformal
dimension of the primary operator that is associated with the state via the
operator-state correspondence, while higher order terms depend on detailed
properties of the operator, such as its OPE with itself and generally involve
many bulk fields. We illustrate the discussion with the holographic
construction of the universal part of the solution for states of two
dimensional CFTs, either on or on . We compute the
1-point function both in the CFT and in the bulk, finding exact agreement. We
comment on the relation with other reconstruction approaches.Comment: 26 pages, 4 figures, v2: comments adde
Time evolution of the chiral phase transition during a spherical expansion
We examine the non-equilibrium time evolution of the hadronic plasma produced
in a relativistic heavy ion collision, assuming a spherical expansion into the
vacuum. We study the linear sigma model to leading order in a large-
expansion. Starting at a temperature above the phase transition, the system
expands and cools, finally settling into the broken symmetry vacuum state. We
consider the proper time evolution of the effective pion mass, the order
parameter , and the particle number distribution. We
examine several different initial conditions and look for instabilities
(exponentially growing long wavelength modes) which can lead to the formation
of disoriented chiral condensates (DCCs). We find that instabilities exist for
proper times which are less than 3 fm/c. We also show that an experimental
signature of domain growth is an increase in the low momentum spectrum of
outgoing pions when compared to an expansion in thermal equilibrium. In
comparison to particle production during a longitudinal expansion, we find that
in a spherical expansion the system reaches the ``out'' regime much faster and
more particles get produced. However the size of the unstable region, which is
related to the domain size of DCCs, is not enhanced.Comment: REVTex, 20 pages, 8 postscript figures embedded with eps
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