1,755 research outputs found
Constant-time distributed dominating set approximation
Abstract.: Finding a small dominating set is one of the most fundamental problems of classical graph theory. In this paper, we present a new fully distributed approximation algorithm based on LP relaxation techniques. For an arbitrary, possibly constant parameter k and maximum node degree , our algorithm computes a dominating set of expected size in rounds. Each node has to send messages of size . This is the first algorithm which achieves a non-trivial approximation ratio in a constant number of round
Effects of 3D Geometries on Cellular Gradient Sensing and Polarization
During cell migration, cells become polarized, change their shape, and move
in response to various internal and external cues. Cell polarization is defined
through the spatio-temporal organization of molecules such as PI3K or small
GTPases, and is determined by intracellular signaling networks. It results in
directional forces through actin polymerization and myosin contractions. Many
existing mathematical models of cell polarization are formulated in terms of
reaction-diffusion systems of interacting molecules, and are often defined in
one or two spatial dimensions. In this paper, we introduce a 3D
reaction-diffusion model of interacting molecules in a single cell, and find
that cell geometry has an important role affecting the capability of a cell to
polarize, or change polarization when an external signal changes direction. Our
results suggest a geometrical argument why more roundish cells can repolarize
more effectively than cells which are elongated along the direction of the
original stimulus, and thus enable roundish cells to turn faster, as has been
observed in experiments. On the other hand, elongated cells preferentially
polarize along their main axis even when a gradient stimulus appears from
another direction. Furthermore, our 3D model can accurately capture the effect
of binding and unbinding of important regulators of cell polarization to and
from the cell membrane. This spatial separation of membrane and cytosol, not
possible to capture in 1D or 2D models, leads to marked differences of our
model from comparable lower-dimensional models.Comment: 31 pages, 7 figure
X-ray astronomy in the new Millenium. A Summary
Recent X-ray observations have had a major impact on topics ranging from
protostars to cosmology. They have also drawn attention to important and
general physical processes that currently limit our understanding of thermal
and nonthermal X-ray sources. These include unmeasured atomic astrophysics data
(wavelengths, oscillator strengths etc.), basic hydromagnetic processes (e.g.
shock structure, reconnection), plasma processes (such as electron-ion
equipartition and heat conduction) and radiative transfer (in disks and
accretion columns). Progress on these problems will probably come from
integrative studies that draw upon observations, throughout the electromagnetic
spectrum, of different classes of source. X-ray observations are also giving a
new perspective on astronomical subjects, like the nature of galactic nuclei
and the evolution of stellar populations. They are contributing to answering
central cosmological questions including the measurement of the matter content
of the universe, understanding its overall luminosity density, describing its
chemical evolution and locating the first luminous objects. X-ray astronomy has
a healthy future with several international space missions under construction
and in development.Comment: 12 page
Malaria parasite detection increases during pregnancy in wild chimpanzees
Background: The diversity of malaria parasites (Plasmodium sp.) infecting chimpanzees (Pan troglodytes) and their close relatedness with those infecting humans is well documented. However, their biology is still largely unexplored and there is a need for baseline epidemiological data. Here, the effect of pregnancy, a well-known risk factor for malaria in humans, on the susceptibility of female chimpanzees to malaria infection was investigated.
Methods: A series of 384 faecal samples collected during 40 pregnancies and 36 post-pregnancies from three habituated groups of wild chimpanzees in the Tai National Park, Cote d'Ivoire, were tested. Samples were tested for malaria parasites by polymerase chain reaction (PCR) and sequencing. Data were analysed using a generalized linear mixed model.
Results: Probability of malaria parasite detection significantly increased towards the end of pregnancy and decreased with the age of the mother.
Conclusions: This study provides evidence that susceptibility to malaria parasite infection increases during pregnancy, and, as shown before, in younger individuals, which points towards similar dynamics of malaria parasite infection in human and chimpanzee populations and raises questions about the effects of such infections on pregnancy outcome and offspring morbidity/mortality
Dynamic Analysis of the Arrow Distributed Protocol
Distributed queuing is a fundamental coordination problem that arises in a variety of applications, including distributed directories, totally ordered multicast, and distributed mutual exclusion. The arrow protocol is a solution to distributed queuing that is based on path reversal on a pre-selected spanning tree of the network. We present a novel and comprehensive competitive analysis of the arrow protocol. We consider the total cost of handling a finite number of queuing requests, which may or may not be issued concurrently, and show that the arrow protocol is -competitive to the optimal queuing protocol, where s and D are the stretch and the diameter, respectively, of the spanning tree. In addition, we show that our analysis is almost tight by proving that for every spanning tree chosen for execution, the arrow protocol is -competitive to the optimal queuing protocol. Our analysis reveals an intriguing connection between the arrow protocol and the nearest neighbor traveling salesperson tour on an appropriately defined grap
Single-Cell Migration in Complex Microenvironments: Mechanics and Signaling Dynamics
Cells are highly dynamic and mechanical automata powered by molecular motors that respond to external cues. Intracellular signaling pathways, either chemical or mechanical, can be activated and spatially coordinated to induce polarized cell states and directional migration. Physiologically, cells navigate through complex microenvironments, typically in three-dimensional (3D) fibrillar networks. In diseases, such as metastatic cancer, they invade across physiological barriers and remodel their local environments through force, matrix degradation, synthesis, and reorganization. Important external factors such as dimensionality, confinement, topographical cues, stiffness, and flow impact the behavior of migrating cells and can each regulate motility. Here, we review recent progress in our understanding of single-cell migration in complex microenvironments.National Cancer Institute (U.S.) (Grant No. 5U01CA177799)National Institutes of Health (U.S.) (Ruth L. Kirschstein National Research Service Award
On Implicit Bias in Overparameterized Bilevel Optimization
Many problems in machine learning involve bilevel optimization (BLO),
including hyperparameter optimization, meta-learning, and dataset distillation.
Bilevel problems consist of two nested sub-problems, called the outer and inner
problems, respectively. In practice, often at least one of these sub-problems
is overparameterized. In this case, there are many ways to choose among optima
that achieve equivalent objective values. Inspired by recent studies of the
implicit bias induced by optimization algorithms in single-level optimization,
we investigate the implicit bias of gradient-based algorithms for bilevel
optimization. We delineate two standard BLO methods -- cold-start and
warm-start -- and show that the converged solution or long-run behavior depends
to a large degree on these and other algorithmic choices, such as the
hypergradient approximation. We also show that the inner solutions obtained by
warm-start BLO can encode a surprising amount of information about the outer
objective, even when the outer parameters are low-dimensional. We believe that
implicit bias deserves as central a role in the study of bilevel optimization
as it has attained in the study of single-level neural net optimization.Comment: ICML 202
Diseño de módulo de control pra el sistema de iluminación y movimiento de personajes del Teatrino Inteligente
Recent X-Ray Observations of SN1986J with ASCA and ROSAT
We present ASCA and ROSAT observations of SN 1986J covering the period 1991
August to 1996 January. From observations with the ROSAT HRI and PSPC, we find
that the 0.5-2.5 keV flux decreased proportional to during this
period; the ASCA data are consistent with this result and extend it to the 2-10
keV band. ASCA spectra from 1994 January and 1996 January are consistent with
thermal emission from a solar metallicity plasma at an equilibrium temperature
kT = 5-7.5 keV, somewhat hotter than that observed from other X-ray supernovae.
These spectra also show a clear Fe K emission line at 6.7 keV with FWHM <
20,000 km/s (90% confidence). This limit on the line width is consistent with
the reverse shock model of Chevalier & Fransson (1994), but does not rule out
the clumpy wind model of Chugai (1993).Comment: 20 pages, 9 postscript figures, latex, uses aastex4.0, submitted to
The Astrophysical Journa
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