1,349 research outputs found
Digraph extremal problems, hypergraph extremal problems, and the densities of graph structures
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
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]
Imaging Sources with Fast and Slow Emission Components
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)
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
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
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
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 is the magnitude of
the negative energy which is transmitted on a timescale . 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
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
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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