1,351 research outputs found
Self-Stabilizing Wavelets and r-Hops Coordination
We introduce a simple tool called the wavelet (or, r-wavelet) scheme.
Wavelets deals with coordination among processes which are at most r hops away
of each other. We present a selfstabilizing solution for this scheme. Our
solution requires no underlying structure and works in arbritrary anonymous
networks, i.e., no process identifier is required. Moreover, our solution works
under any (even unfair) daemon. Next, we use the wavelet scheme to design
self-stabilizing layer clocks. We show that they provide an efficient device in
the design of local coordination problems at distance r, i.e., r-barrier
synchronization and r-local resource allocation (LRA) such as r-local mutual
exclusion (LME), r-group mutual exclusion (GME), and r-Reader/Writers. Some
solutions to the r-LRA problem (e.g., r-LME) also provide transformers to
transform algorithms written assuming any r-central daemon into algorithms
working with any distributed daemon
Scatter of Weak Robots
In this paper, we first formalize the problem to be solved, i.e., the Scatter
Problem (SP). We then show that SP cannot be deterministically solved. Next, we
propose a randomized algorithm for this problem. The proposed solution is
trivially self-stabilizing. We then show how to design a self-stabilizing
version of any deterministic solution for the Pattern Formation and the
Gathering problems
Gracefully Degrading Gathering in Dynamic Rings
Gracefully degrading algorithms [Biely \etal, TCS 2018] are designed to
circumvent impossibility results in dynamic systems by adapting themselves to
the dynamics. Indeed, such an algorithm solves a given problem under some
dynamics and, moreover, guarantees that a weaker (but related) problem is
solved under a higher dynamics under which the original problem is impossible
to solve. The underlying intuition is to solve the problem whenever possible
but to provide some kind of quality of service if the dynamics become
(unpredictably) higher.In this paper, we apply for the first time this approach
to robot networks. We focus on the fundamental problem of gathering a squad of
autonomous robots on an unknown location of a dynamic ring. In this goal, we
introduce a set of weaker variants of this problem. Motivated by a set of
impossibility results related to the dynamics of the ring, we propose a
gracefully degrading gathering algorithm
Optimal Probabilistic Ring Exploration by Asynchronous Oblivious Robots
We consider a team of identical, oblivious, asynchronous mobile robots
that are able to sense (\emph{i.e.}, view) their environment, yet are unable to
communicate, and evolve on a constrained path. Previous results in this weak
scenario show that initial symmetry yields high lower bounds when problems are
to be solved by \emph{deterministic} robots. In this paper, we initiate
research on probabilistic bounds and solutions in this context, and focus on
the \emph{exploration} problem of anonymous unoriented rings of any size. It is
known that robots are necessary and sufficient to solve the
problem with deterministic robots, provided that and are coprime.
By contrast, we show that \emph{four} identical probabilistic robots are
necessary and sufficient to solve the same problem, also removing the coprime
constraint. Our positive results are constructive
Efficient ortho-para conversion of H2 on interstellar grain surfaces
Context: Fast surface conversion between ortho- and para-H2 has been observed
in laboratory studies, and this mechanism has been proposed to play a role in
the control of the ortho-para ratio in the interstellar medium. Observations of
rotational lines of H2 in Photo-Dissociation Regions (PDRs) have indeed found
significantly lower ortho-para ratios than expected at equilibrium. The
mechanisms controlling the balance of the ortho-para ratio in the interstellar
medium thus remain incompletely understood, while this ratio can affect the
thermodynamical properties of the gas (equation of state, cooling function).
Aims: We aim to build an accurate model of ortho-para conversion on dust
surfaces based on the most recent experimental and theoretical results, and to
validate it by comparison to observations of H2 rotational lines in PDRs.
Methods: We propose a statistical model of ortho-para conversion on dust grains
with fluctuating dust temperatures, based on a master equation approach. This
computation is then coupled to full PDR models and compared to PDR
observations. Results: We show that the observations of rotational H2 lines
indicate a high conversion efficiency on dust grains, and that this high
efficiency can be accounted for if taking dust temperature fluctuations into
account with our statistical model of surface conversion. Simpler models
neglecting the dust temperature fluctuations do not reach the high efficiency
deduced from the observations. Moreover, this high efficiency induced by dust
temperature fluctuations is quite insensitive to the values of microphysical
parameters of the model. Conclusions: Ortho-para conversion on grains is thus
an efficient mechanism in most astrophysical conditions that can play a
significant role in controlling the ortho-para ratio.Comment: Accepted in Astronomy & Astrophysic
Surface chemistry in the Interstellar Medium II. formation on dust with random temperature fluctuations
The formation on grains is known to be sensitive to dust
temperature, which is also known to fluctuate for small grain sizes due to
photon absorption. We aim at exploring the consequences of simultaneous
fluctuations of the dust temperature and the adsorbed H-atom population on the
formation rate under the full range of astrophysically relevant
UV intensities and gas conditions. The master equation approach is generalized
to coupled fluctuations in both the grain's temperature and its surface
population and solved numerically. The resolution can be simplified in the case
of the Eley-Rideal mechanism, allowing a fast computation. For the
Langmuir-Hinshelwood mechanism, it remains computationally expensive, and
accurate approximations are constructed. We find the Langmuir-Hinshelwood
mechanism to become an efficient formation mechanism in unshielded photon
dominated region (PDR) edge conditions when taking those fluctuations into
account, despite hot average dust temperatures. It reaches an importance
comparable to the Eley-Rideal mechanism. However, we show that a simpler rate
equation treatment gives qualitatively correct observable results in full cloud
simulations under most astrophysically relevant conditions. Typical differences
are a factor of 2-3 on the intensities of the lines. We
also find that rare fluctuations in cloud cores are sufficient to significantly
reduce the formation efficiency. Our detailed analysis confirms that the usual
approximations used in numerical models are adequate when interpreting
observations, but a more sophisticated statistical analysis is required if one
is interested in the details of surface processes.Comment: 21 pages, 28 figures, accepted in A&
Adhesive factor/rabbit 2, a new fimbrial adhesin and a virulence factor from Escherichia coli O103, a serogroup enteropathogenic for rabbits
Enteropathogenic Escherichia coli-like E. coli strains belonging to serovar O103:K-:H2 and rhamnose-negative biotypes are highly pathogenic diarrhea-inducing strains for weaned European rabbits. We describe here the cloning and sequencing of the major subunit gene of a new fimbrial adhesin, adhesive factor/rabbit 2 (AF/R2), which confers on these strains the ability to attach to rabbit enterocytes and to HeLa cells in a diffuse manner and which is associated with in vivo virulence. The chromosomal operon that encodes functional AF/R2 has been cloned from strain B10. The major subunit gene afr2G, as well as an adjacent open reading frame, afr2H, has been sequenced. The Afr2G protein shows homologies with FaeG and ClpG, which are the respective major subunits of fimbrial adhesin K88 (F4) and afimbrial adhesin CS31A. Plasmid carrying the operon transcomplements an AF/R2-negative TnphoA mutant for its ability to express AF/R2. As a whole, AF/R2 is a new member of the E. coli K88 adhesin family which is associated with virulence and which may serve in the design of vaccines
Deterministic Rendezvous at a Node of Agents with Arbitrary Velocities
We consider the task of rendezvous in networks modeled as undirected graphs.
Two mobile agents with different labels, starting at different nodes of an
anonymous graph, have to meet. This task has been considered in the literature
under two alternative scenarios: weak and strong. Under the weak scenario,
agents may meet either at a node or inside an edge. Under the strong scenario,
they have to meet at a node, and they do not even notice meetings inside an
edge. Rendezvous algorithms under the strong scenario are known for synchronous
agents. For asynchronous agents, rendezvous under the strong scenario is
impossible even in the two-node graph, and hence only algorithms under the weak
scenario were constructed. In this paper we show that rendezvous under the
strong scenario is possible for agents with restricted asynchrony: agents have
the same measure of time but the adversary can arbitrarily impose the speed of
traversing each edge by each of the agents. We construct a deterministic
rendezvous algorithm for such agents, working in time polynomial in the size of
the graph, in the length of the smaller label, and in the largest edge
traversal time.Comment: arXiv admin note: text overlap with arXiv:1704.0888
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