12,475 research outputs found

    Vertex elimination orderings for hereditary graph classes

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    We provide a general method to prove the existence and compute efficiently elimination orderings in graphs. Our method relies on several tools that were known before, but that were not put together so far: the algorithm LexBFS due to Rose, Tarjan and Lueker, one of its properties discovered by Berry and Bordat, and a local decomposition property of graphs discovered by Maffray, Trotignon and Vu\vskovi\'c. We use this method to prove the existence of elimination orderings in several classes of graphs, and to compute them in linear time. Some of the classes have already been studied, namely even-hole-free graphs, square-theta-free Berge graphs, universally signable graphs and wheel-free graphs. Some other classes are new. It turns out that all the classes that we study in this paper can be defined by excluding some of the so-called Truemper configurations. For several classes of graphs, we obtain directly bounds on the chromatic number, or fast algorithms for the maximum clique problem or the coloring problem

    Low rank perturbations and the spectral statistics of pseudointegrable billiards

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    We present an efficient method to solve Schr\"odinger's equation for perturbations of low rank. In particular, the method allows to calculate the level counting function with very little numerical effort. To illustrate the power of the method, we calculate the number variance for two pseudointegrable quantum billiards: the barrier billiard and the right triangle billiard (smallest angle π/5\pi/5). In this way, we obtain precise estimates for the level compressibility in the semiclassical (high energy) limit. In both cases, our results confirm recent theoretical predictions, based on periodic orbit summation.Comment: 4 page

    Hall Conductance of a Two-Dimensional Electron Gas in Periodic Lattice with Triangular Antidots

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    The topic of this contribution is the investigation of quantum states and quantum Hall effect in electron gas subjected to a periodic potential of the lateral lattice. The potential is formed by triangular quantum antidos located on the sites of the square lattice. In a such system the inversion center and the four-fold rotation symmetry are absent. The topological invariants which characterize different magnetic subbands and their Hall conductances are calculated. It is shown that the details of the antidot geometry are crucial for the Hall conductance quantization rule. The critical values of lattice parameters defining the shape of triangular antidots at which the Hall conductance is changed drastically are determined. We demonstrate that the quantum states and Hall conductance quantization law for the triangular antidot lattice differ from the case of the square lattice with cylindrical antidots. As an example, the Hall conductances of magnetic subbands for different antidot geometries are calculated for the case when the number of magnetic flux quanta per unit cell is equal to three.Comment: 6 pages, 5 figure

    Geometric Phase, Curvature, and Extrapotentials in Constrained Quantum Systems

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    We derive an effective Hamiltonian for a quantum system constrained to a submanifold (the constraint manifold) of configuration space (the ambient space) by an infinite restoring force. We pay special attention to how this Hamiltonian depends on quantities which are external to the constraint manifold, such as the external curvature of the constraint manifold, the (Riemannian) curvature of the ambient space, and the constraining potential. In particular, we find the remarkable fact that the twisting of the constraining potential appears as a gauge potential in the constrained Hamiltonian. This gauge potential is an example of geometric phase, closely related to that originally discussed by Berry. The constrained Hamiltonian also contains an effective potential depending on the external curvature of the constraint manifold, the curvature of the ambient space, and the twisting of the constraining potential. The general nature of our analysis allows applications to a wide variety of problems, such as rigid molecules, the evolution of molecular systems along reaction paths, and quantum strip waveguides.Comment: 27 pages with 1 figure, submitted to Phys. Rev.

    Identifying and prioritising services in European terrestrial and freshwater ecosystems

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    Ecosystems are multifunctional and provide humanity with a broad array of vital services. Effective management of services requires an improved evidence base, identifying the role of ecosystems in delivering multiple services, which can assist policy-makers in maintaining them. Here, information from the literature and scientific experts was used to systematically document the importance of services and identify trends in their use and status over time for the main terrestrial and freshwater ecosystems in Europe. The results from this review show that intensively managed ecosystems contribute mostly to vital provisioning services (e.g. agro-ecosystems provide food via crops and livestock, and forests provide wood), while semi-natural ecosystems (e.g. grasslands and mountains) are key contributors of genetic resources and cultural services (e.g. aesthetic values and sense of place). The most recent European trends in human use of services show increases in demand for crops from agro-ecosystems, timber from forests, water flow regulation from rivers, wetlands and mountains, and recreation and ecotourism in most ecosystems, but decreases in livestock production, freshwater capture fisheries, wild foods and virtually all services associated with ecosystems which have considerably decreased in area (e.g. semi-natural grasslands). The condition of the majority of services show either a degraded or mixed status across Europe with the exception of recent enhancements in timber production in forests and mountains, freshwater provision, water/erosion/natural hazard regulation and recreation/ecotourism in mountains, and climate regulation in forests. Key gaps in knowledge were evident for certain services across all ecosystems, including the provision of biochemicals and natural medicines, genetic resources and the regulating services of seed dispersal, pest/disease regulation and invasion resistance

    Commentary on 'What is the point: will screening mammography save my life?' by Keen and Keen

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    Commentary on Keen and Keen 'What is the point: will screening mammography save my life?' BMC Medical Informatics and Decision Making, 200

    PENGARUH KUALITAS PELAYANAN TERHADAP KEPUASAN KONSUMEN DI PT. RAMAYANA MOTOR SUKOHARJO

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    This final project writing aims to find out the satisfaction level the consumers feel on the service given by PT Ramayana Motor Sukoharjo through the dimensions of service quality including Tangible, Reliability, Responsiveness, Assurance and empathy. The problem statement developed in this research is to find out the consumer satisfaction level on the performance given by PT. Ramayana Motor Sukoharjo. Technique of collecting data employed was observation, interview, and questionnaire method. The sampling technique used was simple random sampling with 100 respondents. In this research, technique of discussing used was descriptive analysis one, namely, the one to make a systematic, factual and accurate description about an object studied, while the consumer satisfaction was measured using importance performance analysis. From the research on 100 respondents about the service quality given by PT. Ramayana Motor Sukoharjo, it can be found that the results of consumer assessment on the performance given by the company include: viewed from the importance performance analysis, the highest consumer satisfaction level is in service procedure attribute or factor that will be done quickly, it can be seen from the percentage goodness of fit level of 113.28%, while the smallest attribute is that the employees have knowledge to answer the consumers’ question of 71.15%. The conclusion that can be drawn is that the result of Cartesian diagram of factors becomes the primary priority of PT. Ramayana Motor Sukoharjo and should be done because this factor is considered as important by the company and consumers, including: non strategic and not affordable dealer location, unattractive and untidy physical facility appearance. The recommendation given is that from the location side, the dealer should post the banner in front of its building so that the consumers can find the dealer location easily. From physical facility side, the product arrangement should be adjusted with the types of each product. Keywords: Tangible, Reliability, Responsiveness, Assurance and empathy
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