1,346 research outputs found

    Digraph extremal problems, hypergraph extremal problems, and the densities of graph structures

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    AbstractWe consider extremal problems ‘of Turán type’ for r-uniform ordered hypergraphs, where multiple oriented edges are permitted up to multiplicity q. With any such ‘(r, q)-graph’ Gn we associate an r-linear form whose maximum over the standard (n − 1)-simplex in Rn is called the (graph-) density g(Gn) of Gn. If ex(n, L) is the maximum number of oriented hyperedges in an n-vertex (r, q)-graph not containing a member of L, limn→∞ ex(n, L)/nr is called the extremal density of L. Motivated, in part, from results for ordinary graphs, digraphs, and multigraphs, we establish relations between these two notions

    Probing Transport Theories via Two-Proton Source Imaging

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    Imaging technique is applied to two-proton correlation functions to extract quantitative information about the space-time properties of the emitting source and about the fraction of protons that can be attributed to fast emission mechanisms. These new analysis techniques resolve important ambiguities that bedeviled prior comparisons between measured correlation functions and those calculated by transport theory. Quantitative comparisons to transport theory are presented here. The results of the present analysis differ from those reported previously for the same reaction systems. The shape of the two-proton emitting sources are strongly sensitive to the details about the in-medium nucleon-nucleon cross sections and their density dependence.Comment: 23 pages, 11 figures. Figures are in GIF format. If you need postscript format, please contact: [email protected]

    Fusion Welding of ZrB2-Based Ceramics

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    Imaging Sources with Fast and Slow Emission Components

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    We investigate two-proton correlation functions for reactions in which fast dynamical and slow evaporative proton emission are both present. In such cases, the width of the correlation peak provides the most reliable information about the source size of the fast dynamical component. The maximum of the correlation function is sensitive to the relative yields from the slow and fast emission components. Numerically inverting the correlation function allows one to accurately disentangle fast dynamical from slow evaporative emission and extract details of the shape of the two-proton source.Comment: 13 pages, 4 figure

    (93)Nb NMR spin echo spectroscopy in single crystal NbSe(3)

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    International audienceWe report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe(3) using (93)Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E = (0.96)E(T). The polarizations have a broad distribution extending up to phase angles of order 60 degrees for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable

    Percutaneous endoscopic gastrostomy in children a 5-year experience

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    Introduction. Percutaneous endoscopic gastrostomy (PEG) has been performed on children since 1979. The indications for a PEG are wide ranging and while there are well-established benefits, it remains a procedure with recognised complications.Goals and objectives. The goal of this study was to review our experience with this procedure at a South African paediatric tertiary referral hospital over a 5-year period. The objectives were to review PEGs with regard to patient characteristics, indications, anaesthesia time required and complications.Methods. The study was a retrospective case record review.Results. A total of 70 PEGs were performed. Patients had a mean age of 4 years and 3 months, and a mean weight of 12.2 kg at the time of performing the procedure. The mean anaesthetic time required for performing a PEG was 27 minutes. Fifty-four PEGs (77%) were performed for inability to swallow, 15 (21 %) to improve caloric intake, and 1 (1 %) for continuous enteral feeding. There were no deaths, 5 patients had major complications (6%), and 12 patients (17%) needed antireflux surgery subsequent to the placement of a PEG.Discussion. There is an increasing demand for PEGs at our institution. The indications for a PEG in this series are similar to those reported in other series, although we may be underutilising PEGs to improve caloric intake. Our complication rates compare favourably with those reported in other series. We have, however, identified post-PEG gastro-oesophageal reflux disease as a complication we would like to reduce, and suggest a practical approach to do so

    General relativistic spinning fluids with a modified projection tensor

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    An energy-momentum tensor for general relativistic spinning fluids compatible with Tulczyjew-type supplementary condition is derived from the variation of a general Lagrangian with unspecified explicit form. This tensor is the sum of a term containing the Belinfante-Rosenfeld tensor and a modified perfect-fluid energy-momentum tensor in which the four-velocity is replaced by a unit four-vector in the direction of fluid momentum. The equations of motion are obtained and it is shown that they admit a Friedmann-Robertson-Walker space-time as a solution.Comment: Submitted to General Relativity and Gravitatio

    Motion of Inertial Observers Through Negative Energy

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    Recent research has indicated that negative energy fluxes due to quantum coherence effects obey uncertainty principle-type inequalities of the form |\Delta E|\,{\Delta \tau} \lprox 1\,. Here ∣ΔE∣|\Delta E| is the magnitude of the negative energy which is transmitted on a timescale Δτ\Delta \tau. Our main focus in this paper is on negative energy fluxes which are produced by the motion of observers through static negative energy regions. We find that although a quantum inequality appears to be satisfied for radially moving geodesic observers in two and four-dimensional black hole spacetimes, an observer orbiting close to a black hole will see a constant negative energy flux. In addition, we show that inertial observers moving slowly through the Casimir vacuum can achieve arbitrarily large violations of the inequality. It seems likely that, in general, these types of negative energy fluxes are not constrained by inequalities on the magnitude and duration of the flux. We construct a model of a non-gravitational stress-energy detector, which is rapidly switched on and off, and discuss the strengths and weaknesses of such a detector.Comment: 18pp + 1 figure(not included, available on request), in LATEX, TUPT-93-

    Support of dS/CFT correspondence from perturbations of three dimensional spacetime

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    We discuss the relation between bulk de Sitter three-dimensional spacetime and the corresponding conformal field theory at the boundary, in the framework of the exact quasinormal mode spectrum. We show that the quasinormal mode spectrum corresponds exactly to the spectrum of thermal excitations of Conformal Field Theory at the past boundary I^-, together with the spectrum of the Conformal Field Theory at the future boundary I^+.Comment: minor changes, new version accepted for publication in PL

    The influence of cluster emission and the symmetry energy on neutron-proton spectral double ratios

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    Emissions of free neutrons and protons from the central collisions of 124Sn+124Sn and 112Sn+112Sn reactions are simulated using the Improved Quantum Molecular Dynamics model with two different density dependence of the symmetry energy in the nuclear equation of state. The constructed double ratios of the neutron to proton ratios of the two reaction systems are found to be sensitive to the symmetry terms in the EOS. The effect of cluster formation is examined and found to affect the double ratios mainly in the low energy region. In order to extract better information on symmetry energy with transport models, it is therefore important to have accurate data in the high energy region which also is affected minimally by sequential decays.Comment: 11 pages, 4 figure
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