3,730 research outputs found
Optimal byzantine resilient convergence in oblivious robot networks
Given a set of robots with arbitrary initial location and no agreement on a
global coordinate system, convergence requires that all robots asymptotically
approach the exact same, but unknown beforehand, location. Robots are
oblivious-- they do not recall the past computations -- and are allowed to move
in a one-dimensional space. Additionally, robots cannot communicate directly,
instead they obtain system related information only via visual sensors. We draw
a connection between the convergence problem in robot networks, and the
distributed \emph{approximate agreement} problem (that requires correct
processes to decide, for some constant , values distance
apart and within the range of initial proposed values). Surprisingly, even
though specifications are similar, the convergence implementation in robot
networks requires specific assumptions about synchrony and Byzantine
resilience. In more details, we prove necessary and sufficient conditions for
the convergence of mobile robots despite a subset of them being Byzantine (i.e.
they can exhibit arbitrary behavior). Additionally, we propose a deterministic
convergence algorithm for robot networks and analyze its correctness and
complexity in various synchrony settings. The proposed algorithm tolerates f
Byzantine robots for (2f+1)-sized robot networks in fully synchronous networks,
(3f+1)-sized in semi-synchronous networks. These bounds are optimal for the
class of cautious algorithms, which guarantee that correct robots always move
inside the range of positions of the correct robots
Optimal deterministic ring exploration with oblivious asynchronous robots
We consider the problem of exploring an anonymous unoriented ring of size
by identical, oblivious, asynchronous mobile robots, that are unable to
communicate, yet have the ability to sense their environment and take decisions
based on their local view. Previous works in this weak scenario prove that
must not divide for a deterministic solution to exist. Also, it is known
that the minimum number of robots (either deterministic or probabilistic) to
explore a ring of size is 4. An upper bound of 17 robots holds in the
deterministic case while 4 probabilistic robots are sufficient. In this paper,
we close the complexity gap in the deterministic setting, by proving that no
deterministic exploration is feasible with less than five robots whenever the
size of the ring is even, and that five robots are sufficient for any that
is coprime with five. Our protocol completes exploration in O(n) robot moves,
which is also optimal
Dynamic FTSS in Asynchronous Systems: the Case of Unison
Distributed fault-tolerance can mask the effect of a limited number of
permanent faults, while self-stabilization provides forward recovery after an
arbitrary number of transient fault hit the system. FTSS protocols combine the
best of both worlds since they are simultaneously fault-tolerant and
self-stabilizing. To date, FTSS solutions either consider static (i.e. fixed
point) tasks, or assume synchronous scheduling of the system components. In
this paper, we present the first study of dynamic tasks in asynchronous
systems, considering the unison problem as a benchmark. Unison can be seen as a
local clock synchronization problem as neighbors must maintain digital clocks
at most one time unit away from each other, and increment their own clock value
infinitely often. We present many impossibility results for this difficult
problem and propose a FTSS solution when the problem is solvable that exhibits
optimal fault containment
Universal Loop-Free Super-Stabilization
We propose an univesal scheme to design loop-free and super-stabilizing
protocols for constructing spanning trees optimizing any tree metrics (not only
those that are isomorphic to a shortest path tree). Our scheme combines a novel
super-stabilizing loop-free BFS with an existing self-stabilizing spanning tree
that optimizes a given metric. The composition result preserves the best
properties of both worlds: super-stabilization, loop-freedom, and optimization
of the original metric without any stabilization time penalty. As case study we
apply our composition mechanism to two well known metric-dependent spanning
trees: the maximum-flow tree and the minimum degree spanning tree
RoboCast: Asynchronous Communication in Robot Networks
This paper introduces the \emph{RoboCast} communication abstraction. The
RoboCast allows a swarm of non oblivious, anonymous robots that are only
endowed with visibility sensors and do not share a common coordinate system, to
asynchronously exchange information. We propose a generic framework that covers
a large class of asynchronous communication algorithms and show how our
framework can be used to implement fundamental building blocks in robot
networks such as gathering or stigmergy. In more details, we propose a RoboCast
algorithm that allows robots to broadcast their local coordinate systems to
each others. Our algorithm is further refined with a local collision avoidance
scheme. Then, using the RoboCast primitive, we propose algorithms for
deterministic asynchronous gathering and binary information exchange
Stabilizing data-link over non-FIFO channels with optimal fault-resilience
Self-stabilizing systems have the ability to converge to a correct behavior
when started in any configuration. Most of the work done so far in the
self-stabilization area assumed either communication via shared memory or via
FIFO channels. This paper is the first to lay the bases for the design of
self-stabilizing message passing algorithms over unreliable non-FIFO channels.
We propose a fault-send-deliver optimal stabilizing data-link layer that
emulates a reliable FIFO communication channel over unreliable capacity bounded
non-FIFO channels
Bounds for self-stabilization in unidirectional networks
A distributed algorithm is self-stabilizing if after faults and attacks hit
the system and place it in some arbitrary global state, the systems recovers
from this catastrophic situation without external intervention in finite time.
Unidirectional networks preclude many common techniques in self-stabilization
from being used, such as preserving local predicates. In this paper, we
investigate the intrinsic complexity of achieving self-stabilization in
unidirectional networks, and focus on the classical vertex coloring problem.
When deterministic solutions are considered, we prove a lower bound of
states per process (where is the network size) and a recovery time of at
least actions in total. We present a deterministic algorithm with
matching upper bounds that performs in arbitrary graphs. When probabilistic
solutions are considered, we observe that at least states per
process and a recovery time of actions in total are required (where
denotes the maximal degree of the underlying simple undirected graph).
We present a probabilistically self-stabilizing algorithm that uses
states per process, where is a parameter of the
algorithm. When , the algorithm recovers in expected
actions. When may grow arbitrarily, the algorithm
recovers in expected O(n) actions in total. Thus, our algorithm can be made
optimal with respect to space or time complexity
Synthesis and Antileishmanial Activity of 1,2,4,5-Tetraoxanes against Leishmania donovani
A chemically diverse range of novel tetraoxanes was synthesized and evaluated in vitro against intramacrophage amastigote forms of Leishmania donovani. All 15 tested tetraoxanes displayed activity, with IC50 values ranging from 2 to 45 µm. The most active tetraoxane, compound LC140, exhibited an IC50 value of 2.52 ± 0.65 µm on L. donovani intramacrophage amastigotes, with a selectivity index of 13.5. This compound reduced the liver parasite burden of L. donovani-infected mice by 37% after an intraperitoneal treatment at 10 mg/kg/day for five consecutive days, whereas miltefosine, an antileishmanial drug in use, reduced it by 66%. These results provide a relevant basis for the development of further tetraoxanes as effective, safe, and cheap drugs against leishmaniasis.This research was funded by Fundação para a Ciência e a Tecnologia (FCT), and FEDER/COMPETE 2020-UE,
through projects UID/Multi/04326/2019 (Centre of Marine Sciences-CCMAR) and PTDC/MAR-BIO/4132/2014.info:eu-repo/semantics/publishedVersio
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