10,018 research outputs found

    Tight Bounds for Black Hole Search with Scattered Agents in Synchronous Rings

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    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

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    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

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    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

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    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

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    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

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    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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