13,582 research outputs found
Edge coating of flat wires
An apparatus and technique is described for the coating of the edge surfaces of flat ribbon conductors with an adherent coating of a dielectric insulating material. Means for passing the ribbon conductors between a pair of generally axially aligned rollers is provided. The edge surfaces of the conductor are disposed adjacent to and generally tangentially to the confronting surfaces of the roller so as to form a fillet of dielectric material along the edge surface of the conductor
L\'evy-like behavior in deterministic models of intelligent agents exploring heterogeneous environments
Many studies on animal and human movement patterns report the existence of
scaling laws and power-law distributions. Whereas a number of random walk
models have been proposed to explain observations, in many situations
individuals actually rely on mental maps to explore strongly heterogeneous
environments. In this work we study a model of a deterministic walker, visiting
sites randomly distributed on the plane and with varying weight or
attractiveness. At each step, the walker minimizes a function that depends on
the distance to the next unvisited target (cost) and on the weight of that
target (gain). If the target weight distribution is a power-law, , in some range of the exponent , the foraging medium induces
movements that are similar to L\'evy flights and are characterized by
non-trivial exponents. We explore variations of the choice rule in order to
test the robustness of the model and argue that the addition of noise has a
limited impact on the dynamics in strongly disordered media.Comment: 15 pages, 7 figures. One section adde
Faster k-Medoids Clustering: Improving the PAM, CLARA, and CLARANS Algorithms
Clustering non-Euclidean data is difficult, and one of the most used
algorithms besides hierarchical clustering is the popular algorithm
Partitioning Around Medoids (PAM), also simply referred to as k-medoids. In
Euclidean geometry the mean-as used in k-means-is a good estimator for the
cluster center, but this does not hold for arbitrary dissimilarities. PAM uses
the medoid instead, the object with the smallest dissimilarity to all others in
the cluster. This notion of centrality can be used with any (dis-)similarity,
and thus is of high relevance to many domains such as biology that require the
use of Jaccard, Gower, or more complex distances.
A key issue with PAM is its high run time cost. We propose modifications to
the PAM algorithm to achieve an O(k)-fold speedup in the second SWAP phase of
the algorithm, but will still find the same results as the original PAM
algorithm. If we slightly relax the choice of swaps performed (at comparable
quality), we can further accelerate the algorithm by performing up to k swaps
in each iteration. With the substantially faster SWAP, we can now also explore
alternative strategies for choosing the initial medoids. We also show how the
CLARA and CLARANS algorithms benefit from these modifications. It can easily be
combined with earlier approaches to use PAM and CLARA on big data (some of
which use PAM as a subroutine, hence can immediately benefit from these
improvements), where the performance with high k becomes increasingly
important.
In experiments on real data with k=100, we observed a 200-fold speedup
compared to the original PAM SWAP algorithm, making PAM applicable to larger
data sets as long as we can afford to compute a distance matrix, and in
particular to higher k (at k=2, the new SWAP was only 1.5 times faster, as the
speedup is expected to increase with k)
Exact results for the Barabasi model of human dynamics
Human activity patterns display a bursty dynamics, with interevent times
following a heavy tailed distribution. This behavior has been recently shown to
be rooted in the fact that humans assign their active tasks different
priorities, a process that can be modeled as a priority queueing system [A.-L.
Barabasi, Nature 435, 207 (2005)]. In this work we obtain exact results for the
Barabasi model with two tasks, calculating the priority and waiting time
distribution of active tasks. We demonstrate that the model has a singular
behavior in the extremal dynamics limit, when the highest priority task is
selected first. We find that independently of the selection protocol, the
average waiting time is smaller or equal to the number of active tasks, and
discuss the asymptotic behavior of the waiting time distribution. These results
have important implications for understanding complex systems with extremal
dynamics.Comment: 4 pages, 4 figures, revte
Three-Dimensional Magnetohydrodynamic Simulations of Spherical Accretion
We present three-dimensional numerical magnetohydrodynamic simulations of
radiatively inefficient spherical accretion onto a black hole. The simulations
are initialized with a Bondi flow, and with a weak, dynamically unimportant,
large-scale magnetic field. The magnetic field is amplified as the gas flows
in. When the magnetic pressure approaches equipartition with the gas pressure,
the field begins to reconnect and the gas is heated up. The heated gas is
buoyant and moves outward, causing line stretching of the frozen-in magnetic
field. This leads to further reconnection, and more heating and
buoyancy-induced motions, so that the flow makes a transition to a state of
self-sustained convection. The radial structure of the flow changes
dramatically from its initial Bondi profile, and the mass accretion rate onto
the black hole decreases significantly. Motivated by the numerical results, we
develop a simplified analytical model of a radiatively inefficient spherical
flow in which convective transport of energy to large radii plays an important
role. In this "convection-dominated Bondi flow" the accretion velocity is
highly subsonic and the density varies with radius as ~R^{-1/2} rather than the
standard Bondi scaling ~R^{-3/2}. We estimate that the mass accretion rate onto
the black hole is significantly less than the Bondi accretion rate.
Convection-dominated Bondi flows may be relevant for understanding many
astrophysical phenomena, e.g. post-supernova fallback and radiatively
inefficient accretion onto supermassive black holes, stellar-mass black holes
and neutron stars.Comment: 23 pages, 6 figures, submitted to Ap
Improved Si:As BIBIB (Back-Illuminated Blocked-Impurity-Band) hybrid arrays
Results of a program to increase the short wavelength (less than 10 microns) detective quantum efficiency, eta/beta, of Si:As Impurity Band Conduction arrays are presented. The arrays are epitaxially grown Back-Illuminated Blocked (BIB) Impurity-Band (BIBIB) 10x50 detectors bonded to switched-FET multiplexers. It is shown that the 4.7 microns detective quantum efficiency increases proportionately with the thickness of the infrared active layer. A BIB array with a thick active layer, designed for low dark current, exhibits eta/beta = 7 to 9 percent at 4.7 microns for applied bias voltages between 3 and 5 V. The product of quantum efficiency and photoelectric gain, etaG, increases from 0.3 to 2.5 as the voltage increases from 3 to 5 V. Over this voltage range, the dark current increases from 8 to 120 e(-)s(-1) at a device temperature of 4.2 K and is under 70 e(-)s(-1) for all voltages at 2 K. Because of device gain, the effective dark current (equivalent photon rate) is less than 3 e(-)s(-1) under all operating conditions. The effective read noise (equivalent photon noise) is found to be less than 12 electrons under all operating conditions and for integration times between 0.05 and 100 seconds
Three-body recombination of ultra-cold atoms to a weakly bound level
We discuss three-body recombination of ultra-cold atoms to a weakly bound
level. In this case, characterized by large and positive scattering length
for pair interaction, we find a repulsive effective potential for three-body
collisions, which strongly reduces the recombination probability and makes
simple Jastrow-like approaches absolutely inadequate. In the zero temperature
limit we obtain a universal relation, independent of the detailed shape of the
interaction potential, for the (event) rate constant of three-body
recombination: , where is the atom mass.Comment: 10 pages, 3 Postscript figure
Effects of cover crops on phosphatase activity in a clay arable soil in the UK
The effect of five cover crop species (radish, buckwheat, vetch, phacelia and oat) alongside an un-cropped control, on the activity and persistence of soil acid and alkaline phosphatase activity was investigated. There was no effect on alkaline phosphatase activity at the time of cover crop incorporation (March), but by the point of maturation of the following oat cash crop (June) significant differences were detected, with the greatest activity following an oat cover crop. Acid phosphatase activity showed species-related significant differences at both sampling dates, with the magnitude increasing by June. Again, plots following an oat cover crop showed the greatest activity, followed by phacelia. This has shown that soil phosphatase enzymes are affected by the presence of a cover crop, that this effect is apparently species-dependent – and not dependent on the amount of biomass from the cover crop – and that cover crops could be a potential means to enhance soil phosphorus cycling
Unresolved H-Alpha Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced H-Alpha emission at |b| > 10
degrees subtending approximately 1 degree or less on the Wisconsin H-Alpha
Mapper (WHAM) sky survey. These high latitude ``WHAM point sources'' have
H-Alpha fluxes of 10^{-11} to 10^{-9} erg cm^-2 s^-1, radial velocities within
about 70 km/s of the LSR, and line widths that range from less than 20 km/s to
about 80 km/s (FWHM). Twenty nine of these enhancements are not identified with
either cataloged nebulae or hot stars and appear to have kinematic properties
that differ from those observed for planetary nebulae. Another 14 enhancements
are near hot evolved low mass stars that had no previously reported detections
of associated nebulosity. The remainder of the enhancements are cataloged
planetary nebulae and small, high latitude H II regions surrounding massive O
and early B stars.Comment: 22 pages, 8 figures, 2 tables, to appear in Feb. 2005 A
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