13,738 research outputs found
A map on the space of rational functions
We describe dynamical properties of a map defined on the space
of rational functions. The fixed points of are classified and
the long time behavior of a subclass is described in terms of Eulerian
polynomials
Technical management techniques for identification and control of industrial safety and pollution hazards
Constructive recommendations are suggested for pollution problems from offshore energy resources industries on outer continental shelf. Technical management techniques for pollution identification and control offer possible applications to space engineering and management
Radar Performance Improvement
The AN/APQ-153 fire control radar modified to provide angle tracking was evaluated for improved performance. The frequency agile modifications are discussed along with the range-rate improvement modifications, and the radar to computer interface. A parametric design and comparison of noncoherent and coherent radar systems are presented. It is shown that the shuttle rendezvous range and range-rate requirements can be made by a Ku-Band noncoherent pulse radar
7-Li(p,n) Nuclear Data Library for Incident Proton Energies to 150 MeV
We describe evaluation methods that make use of experimental data, and
nuclear model calculations, to develop an ENDF-formatted data library for the
reaction p + Li7 for incident protons with energies up to 150 MeV. The
important 7-Li(p,n_0) and 7-Li(p,n_1) reactions are evaluated from the
experimental data, with their angular distributions represented using Lengendre
polynomial expansions. The decay of the remaining reaction flux is estimated
from GNASH nuclear model calculations. The evaluated ENDF-data are described in
detail, and illustrated in numerous figures. We also illustrate the use of
these data in a representative application by a radiation transport simulation
with the code MCNPX.Comment: 11 pages, 8 figures, LaTeX, submitted to Proc. 2000 ANS/ENS
International Meeting, Nuclear Applications of Accelerator Technology
(AccApp00), November 12-16, Washington, DC, US
Madym: A C++ toolkit for quantitative DCE-MRI analysis
In dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) a sequence of MRI images are acquired to measure the passage of a contrast-agent within a tissue of interest.
Quantitative DCE-MRI (DCE-MRI), in which one or more tracer-kinetic models are fitted to the contrast-agent concentration time-series, enables the estimation of clinically useful parameters of tissue microvasculature (Tofts et al., 1999).
Madym is a C++ toolkit for quantitative DCE-MRI analysis developed at the University of Manchester. It comprises a set of command line tools and a graphical user-interface based on an extendable C++ library. It is cross-platform, and requires few external libraries to build from source. Pre-built binaries (with all dependencies included) for Windows, MacOS and Linux are available so that Madym can be installed directly for users not wanting to or unable to compile the C++ source themselves. We have also developed complementary interfaces in Matlab (available in a separate open-source repository (M. Berks, 2021b)) and python (integrated with the main toolkit), that allow the flexibility of developing in those scripting languages, while allowing C++ to do the heavy-duty computational work of tracer-kinetic model fittin
The measurement of low pay in the UK labour force survey
Consideration of the National Minimum Wage requires estimates of the distribution of hourly pay. The UK Labour Force Survey (LFS) is a key source of such estimates. The approach most frequently adopted by researchers has been to measure hourly earnings from several questions on pay and hours. The Office for National Statistics is now applying a new approach, based on an alternative more direct measurement introduced in March 1999. These two measures do not produce identical values and this paper investigates sources of discrepancies and concludes that the new variable is more accurate. The difficulty with using the new variable is that it is only available on a subset of respondents. An approach is developed in which missing values of the new variable are replaced by imputed values. The assumptions underlying this imputation approach and results of applying it to LFS data are presented. The relation to weighting approaches is also discussed
Fundamental Discreteness Limitations of Cosmological N-Body Clustering Simulations
We explore some of the effects that discreteness and two-body scattering may
have on N-body simulations with ``realistic'' cosmological initial conditions.
We use an identical subset of particles from the initial conditions for a
Particle-Mesh (PM) calculation as the initial conditions for a variety
PM and Tree code runs. We investigate the effect of mass resolution (the
mean interparticle separation) since most ``high resolution'' codes only have
high resolution in gravitational force. The phase-insensitive two--point
statistics, such as the power spectrum (autocorrelation) are somewhat affected
by these variations, but phase-sensitive statistics show greater differences.
Results converge at the mean interparticle separation scale of the lowest
mass-resolution code. As more particles are added, but the force resolution is
held constant, the PM and the Tree runs agree more and more strongly with
each other and with the PM run which had the same initial conditions. This
shows high particle density is necessary for correct time evolution, since many
different results cannot all be correct. However, they do not so converge to a
PM run which continued the fluctuations to small scales. Our results show that
ignoring them is a major source of error on comoving scales of the missing
wavelengths. This can be resolved by putting in a high particle density. Since
the codes never agree well on scales below the mean comoving interparticle
separation, we find little justification for quantitative predictions on this
scale. Some measures vary by 50%, but others can be off by a factor of three or
more. Our results suggest possible problems with the density of galaxy halos,
formation of early generation objects such as QSO absorber clouds, etc.Comment: Revised version to be published in Astrophysical Journal. One figure
changed; expanded discussion, more information on code parameters. Latex, 44
pages, including 19 figures. Higher resolution versions of Figures 10-15
available at: ftp://kusmos.phsx.ukans.edu/preprints/nbod
Dynamic Singularities in Cooperative Exclusion
We investigate cooperative exclusion, in which the particle velocity can be
an increasing function of the density. Within a hydrodynamic theory, an initial
density upsteps and downsteps can evolve into: (a) shock waves, (b) continuous
compression or rarefaction waves, or (c) a mixture of shocks and continuous
waves. These unusual phenomena arise because of an inflection point in the
current versus density relation. This anomaly leads to a group velocity that
can either be an increasing or a decreasing function of the density on either
side of these wave singularities.Comment: 4 pages, 4 figures, 2 column revtex 4-1 format; version 2:
substantially rewritten and put in IOP format, mail results unchanged;
version 3: minor changes, final version for publication in JSTA
Dense cores in the L1630 molecular cloud: discovering new protostars with SCUBA
Maps of the 450 micron and 850 micron dust continuum emission from three
star-forming condensations within the Lynds 1630 molecular cloud, made with the
SCUBA bolometer array, reveal the presence of four new submillimetre sources,
each of a few solar masses, two of which are probably Class I, two Class 0, as
well as several sources whose existence was previously known. The sources are
located in filaments and appear elongated when observed at 450 microns. They
likely have dust temperatures in the range 10 to 20 K, in good agreement with
previous ammonia temperature estimates. Attempts to fit their structures with
power-law and Gaussian density distributions suggest that the central
distribution is flatter than expected for a simple singular isothermal sphere.
Although the statistics are poor, our results suggest that the ratio of
`protostellar core' mass to total virial mass may be similar for both large and
small condensations.Comment: 10 pages, 9 figures, uses mn.st
Vortex configurations and critical parameters in superconducting thin films containing antidot arrays: Nonlinear Ginzburg-Landau theory
Using the non-linear Ginzburg-Landau (GL) theory, we obtain the possible
vortex configurations in superconducting thin films containing a square lattice
of antidots. The equilibrium structural phase diagram is constructed which
gives the different ground-state vortex configurations as function of the size
and periodicity of the antidots for a given effective GL parameter
. Giant-vortex states, combination of giant- and multi-vortex
states, as well as symmetry imposed vortex-antivortex states are found to be
the ground state for particular geometrical parameters of the sample. The
antidot occupation number is calculated as a function of related
parameters and comparison with existing expressions for the saturation number
and with experimental results is given. For a small radius of antidots a
triangular vortex lattice is obtained, where some of the vortices are pinned by
the antidots and some of them are located between them. Transition between the
square pinned and triangular vortex lattices is given for different values of
the applied field. The enhanced critical current at integer and rational
matching fields is found, where the level of enhancement at given magnetic
field directly depends on the vortex-occupation number of the antidots. For
certain parameters of the antidot lattice and/or temperature the critical
current is found to be larger for higher magnetic fields.
Superconducting/normal phase boundary exhibits different regimes as
antidots are made larger, and we transit from a plain superconducting film to a
thin-wire superconducting network. Presented results are in good agreement with
available experiments and suggest possible new experiments.Comment: 15 pages and 20 figure
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