10,018 research outputs found
Tight Bounds for Black Hole Search with Scattered Agents in Synchronous Rings
We study the problem of locating a particularly dangerous node, the so-called
black hole in a synchronous anonymous ring network with mobile agents. A black
hole is a harmful stationary process residing in a node of the network and
destroying destroys all mobile agents visiting that node without leaving any
trace. We consider the more challenging scenario when the agents are identical
and initially scattered within the network. Moreover, we solve the problem with
agents that have constant-sized memory and carry a constant number of identical
tokens, which can be placed at nodes of the network. In contrast, the only
known solutions for the case of scattered agents searching for a black hole,
use stronger models where the agents have non-constant memory, can write
messages in whiteboards located at nodes or are allowed to mark both the edges
and nodes of the network with tokens. This paper solves the problem for ring
networks containing a single black hole. We are interested in the minimum
resources (number of agents and tokens) necessary for locating all links
incident to the black hole. We present deterministic algorithms for ring
topologies and provide matching lower and upper bounds for the number of agents
and the number of tokens required for deterministic solutions to the black hole
search problem, in oriented or unoriented rings, using movable or unmovable
tokens
Black Hole Search with Finite Automata Scattered in a Synchronous Torus
We consider the problem of locating a black hole in synchronous anonymous
networks using finite state agents. A black hole is a harmful node in the
network that destroys any agent visiting that node without leaving any trace.
The objective is to locate the black hole without destroying too many agents.
This is difficult to achieve when the agents are initially scattered in the
network and are unaware of the location of each other. Previous studies for
black hole search used more powerful models where the agents had non-constant
memory, were labelled with distinct identifiers and could either write messages
on the nodes of the network or mark the edges of the network. In contrast, we
solve the problem using a small team of finite-state agents each carrying a
constant number of identical tokens that could be placed on the nodes of the
network. Thus, all resources used in our algorithms are independent of the
network size. We restrict our attention to oriented torus networks and first
show that no finite team of finite state agents can solve the problem in such
networks, when the tokens are not movable. In case the agents are equipped with
movable tokens, we determine lower bounds on the number of agents and tokens
required for solving the problem in torus networks of arbitrary size. Further,
we present a deterministic solution to the black hole search problem for
oriented torus networks, using the minimum number of agents and tokens
Gravitational wave astronomy - astronomy of the 21st century
An enigmatic prediction of Einstein's general theory of relativity is
gravitational waves. With the observed decay in the orbit of the Hulse-Taylor
binary pulsar agreeing within a fraction of a percent with the theoretically
computed decay from Einstein's theory, the existence of gravitational waves was
firmly established. Currently there is a worldwide effort to detect
gravitational waves with interferometric gravitational wave observatories or
detectors and several such detectors have been built or being built. The
initial detectors have reached their design sensitivities and now the effort is
on to construct advanced detectors which are expected to detect gravitational
waves from astrophysical sources. The era of gravitational wave astronomy has
arrived. This article describes the worldwide effort which includes the effort
on the Indian front - the IndIGO project -, the principle underlying
interferometric detectors both on ground and in space, the principal noise
sources that plague such detectors, the astrophysical sources of gravitational
waves that one expects to detect by these detectors and some glimpse of the
data analysis methods involved in extracting the very weak gravitational wave
signals from detector noise.Comment: The contents of this article were finalised few months ago. The
discussion in the article pertains to the situation prevailing at that tim
Applications of Genetic Algorithms to a Variety of Problems in Physics and Astronomy
Genetic algorithms are search techniques that borrow ideas from the biological process of evolution. By means of natural selection, genetic algorithms can be employed as robust numerical optimizers on problems that would normally be extremely problematic due to ill-behaved search spaces. The genetic algorithm has an advantage in that it is a global optimization strategy, as opposed to more conventional methods, which will often terminate at local maxima.
The success and resourcefulness of genetic algorithms as problem-solving strategies are quickly gaining recognition among researchers of diverse areas of study. In this thesis I elaborate on applications of a genetic algorithm to several problems in physics and astronomy.
First, the concepts behind functional optimization are discussed, as well as several computational strategies for locating optima. The basic ideas behind genetic algorithms and their operations are then outlined, as well as advantages and disadvantages of the genetic algorithm over the previously discussed optimization techniques. Then the results of several applications of a genetic algorithm are discussed. The majority are relatively simple problems (involving the fitting of only one or two parameters) that nicely illustrate the genetic algorithm’s approach to optimization of “fitness,” and its ability to reproduce familiar results. The last two problems discussed are non-trivial and demonstrate the genetic algorithm’s robustness. The first of these was the calculation of the mass of the radio source Sagittarius A*, believed to be a supermassive black hole at the center of the Milky Way, which required that the genetic algorithm find several orbital elements associated with an orbiting star. The results obtained with the genetic algorithm were in good agreement with those obtained by Genzel et al [19]. Then discussed was the problem of parametrization of thermonuclear reaction rates. This problem is especially interesting because attempts at fitting several rates prior to the implementation of the genetic algorithm proved to be unsuccessful. Some of the rates varied with temperature over many orders of magnitude, and required the genetic algorithm to find as many as twenty-eight parameters. A relatively good fit was obtained for all of the rates.
In the applications of genetic algorithms discussed in this thesis, it has been found that they can outperform conventional optimization strategies for difficult, multidimensional problems, and can perform at least as well as conventional methods when applied to more trivial problems
Rotating Electromagnetic Waves in Toroid-Shaped Regions
Electromagnetic waves, solving the full set of Maxwell equations in vacuum,
are numerically computed. These waves occupy a fixed bounded region of the
three dimensional space, topologically equivalent to a toroid. Thus, their
fluid dynamics analogs are vortex rings. An analysis of the shape of the
sections of the rings, depending on the angular speed of rotation and the major
diameter, is carried out. Successively, spherical electromagnetic vortex rings
of Hill's type are taken into consideration. For some interesting peculiar
configurations, explicit numerical solutions are exhibited.Comment: 27 pages, 40 figure
Snapshot coronagraphy with an interferometer in space
Diluted arrays of many optical apertures will be able to provide h
igh-resolution snapshot images if the beams are combined according to the
densified-pupil scheme. We show that the same principle can also provide
coronagraphic images, for detecting faint sources near a bright unresolved one.
Recent refinements of coronagraphic techniques, i.e. the use of a phase mask,
active apodization and dark-speckle analysis, are also applicable for enhanced
contrast. Implemented in the form of a proposed 50-500m Exo-Earth Discoverer
array in space, the principle can serve to detect Earth-like exo-planets in the
infra-red. It can also provide images of faint nebulosity near stars, active
galactic nuclei and quasars. Calculations indicate that exo-planets are
detectable amidst the zodiacal and exo-zodiacal emission faster than with a
Bracewell array of equivalent area, a consequence of the spatial selectivity in
the image.Comment: 23 pages, 10 figures, to appear in Icaru
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