8,763 research outputs found
Competitively tight graphs
The competition graph of a digraph is a (simple undirected) graph which
has the same vertex set as and has an edge between two distinct vertices
and if and only if there exists a vertex in such that
and are arcs of . For any graph , together with sufficiently
many isolated vertices is the competition graph of some acyclic digraph. The
competition number of a graph is the smallest number of such
isolated vertices. Computing the competition number of a graph is an NP-hard
problem in general and has been one of the important research problems in the
study of competition graphs. Opsut [1982] showed that the competition number of
a graph is related to the edge clique cover number of the
graph via . We first show
that for any positive integer satisfying , there
exists a graph with and characterize a graph
satisfying . We then focus on what we call
\emph{competitively tight graphs} which satisfy the lower bound, i.e.,
. We completely characterize the competitively tight
graphs having at most two triangles. In addition, we provide a new upper bound
for the competition number of a graph from which we derive a sufficient
condition and a necessary condition for a graph to be competitively tight.Comment: 10 pages, 2 figure
Inhomogeneous Low Frequency Spin Dynamics in La_{1.65}Eu_{0.2}Sr_{0.15}CuO_4
We report Cu and La nuclear magnetic resonance (NMR) measurements in the
title compound that reveal an inhomogeneous glassy behavior of the spin
dynamics. A low temperature peak in the La spin lattice relaxation rate and the
``wipeout'' of Cu intensity both arise from these slow electronic spin
fluctuations that reveal a distribution of activation energies. Inhomogeneous
slowing of spin fluctuations appears to be a general feature of doped lanthanum
cuprate.Comment: 4 pages, 2 figures. Very slight modifications to figure
A Calculation of Baryon Diffusion Constant in Hot and Dense Hadronic Matter Based on an Event Generator URASiMA
We evaluate thermodynamical quantities and transport coefficients of a dense
and hot hadronic matter based on an event generator URASiMA (Ultra-Relativistic
AA collision Simulator based on Multiple Scattering Algorithm). The statistical
ensembles in equilibrium with fixed temperature and chemical potential are
generated by imposing periodic boundary condition to the simulation of URASiMA,
where energy density and baryon number density is conserved. Achievement of the
thermal equilibrium and the chemical equilibrium are confirmed by the common
value of slope parameter in the energy distributions and the saturation of the
numbers of contained particles, respectively. By using the generated ensembles,
we investigate the temperature dependence and the chemical potential dependence
of the baryon diffusion constant of a dense and hot hadronic matter.Comment: 15 pages, 5 figures, LaTeX2
Flocculation and aggregation in a microgravity environment (FAME)
An experiment to study flocculation phenomena in the constrained microgravity environment of a space shuttle or space station is described. The small size and light weight experiment easily fits in a Spacelab Glovebox. Using an integrated fiber optic dynamic light scattering (DLS) system we obtain high precision particle size measurements from dispersions of colloidal particles within seconds, needs no onboard optical alignment, no index matching fluid, and offers sample mixing and shear melting capabilities to study aggregation (flocculation and coagulation) phenomena under both quiescent and controlled agitation conditions. The experimental system can easily be adapted for other microgravity experiments requiring the use of DLS. Preliminary results of ground-based study are reported
Speckle-visibility spectroscopy: A tool to study time-varying dynamics
We describe a multispeckle dynamic light scattering technique capable of
resolving the motion of scattering sites in cases that this motion changes
systematically with time. The method is based on the visibility of the speckle
pattern formed by the scattered light as detected by a single exposure of a
digital camera. Whereas previous multispeckle methods rely on correlations
between images, here the connection with scattering site dynamics is made more
simply in terms of the variance of intensity among the pixels of the camera for
the specified exposure duration. The essence is that the speckle pattern is
more visible, i.e. the variance of detected intensity levels is greater, when
the dynamics of the scattering site motion is slow compared to the exposure
time of the camera. The theory for analyzing the moments of the spatial
intensity distribution in terms of the electric field autocorrelation is
presented. It is demonstrated for two well-understood samples, a colloidal
suspension of Brownian particles and a coarsening foam, where the dynamics can
be treated as stationary. However, the method is particularly appropriate for
samples in which the dynamics vary with time, either slowly or rapidly, limited
only by the exposure time fidelity of the camera. Potential applications range
from soft-glassy materials, to granular avalanches, to flowmetry of living
tissue.Comment: review - theory and experimen
Numerical approaches to life cycle interpretation: the case of the Ecoinvent'96 database
Wetensch. publicatieInstitute of Environmental Science
Timber Marketing in a Revitalised North Queensland Forest Industry: Overview of Major Issues
As the harvest of timber from native forests has contracted in north Queensland, the traditional supply chain from forest to final consumer has to a large extent broken down. As a result, landholders and non-farmer investors cannot assume that timber markets will automatically exist when they have trees ready for harvest. A wide variety of research has been carried out which is relevant to timber marketing in north Queensland. Surveys of timber millers, cabinet-makers and their staff, and purchasers of timber products, as well as financial modeling of timber milling, have been used to derive information about timber markets in north Queensland. Research has been undertaken on supply chain and market analysis, attitudes of timber processors and final consumers, institutional arrangements for market facilitation, and market development
A New Scintillator Tile/Fiber Preshower Detector for the CDF Central Calorimeter
A detector designed to measure early particle showers has been installed in
front of the central CDF calorimeter at the Tevatron. This new preshower
detector is based on scintillator tiles coupled to wavelength-shifting fibers
read out by multi-anode photomultipliers and has a total of 3,072 readout
channels. The replacement of the old gas detector was required due to an
expected increase in instantaneous luminosity of the Tevatron collider in the
next few years. Calorimeter coverage, jet energy resolution, and electron and
photon identification are among the expected improvements. The final detector
design, together with the R&D studies that led to the choice of scintillator
and fiber, mechanical assembly, and quality control are presented. The detector
was installed in the fall 2004 Tevatron shutdown and started collecting
colliding beam data by the end of the same year. First measurements indicate a
light yield of 12 photoelectrons/MIP, a more than two-fold increase over the
design goals.Comment: 5 pages, 10 figures (changes are minor; this is the final version
published in IEEE-Trans.Nucl.Sci.
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