2,946 research outputs found
Hysteretic optimization for the Sherrington-Kirkpatrick spin glass
Hysteretic optimization is a heuristic optimization method based on the
observation that magnetic samples are driven into a low energy state when
demagnetized by an oscillating magnetic field of decreasing amplitude. We show
that hysteretic optimization is very good for finding ground states of
Sherrington-Kirkpatrick spin glass systems. With this method it is possible to
get good statistics for ground state energies for large samples of systems
consisting of up to about 2000 spins. The way we estimate error rates may be
useful for some other optimization methods as well. Our results show that both
the average and the width of the ground state energy distribution converges
faster with increasing size than expected from earlier studies.Comment: Physica A, accepte
Native Tallgrass Prairie Remnants of Eastern Nebraska: Floristics and Effects of Management, Topography, Size, and Season of Evaluation.
Ten eastern Nebraska tallgrass prairie remnants, varying in size from 1-18 ha, were evaluated for floristic composition at four times during the 1979 growing season. A total of 153 species were recorded of which seven dominated: Andropogon gerardii, A. scoparius, Bromus inermis, Ceanothus americanus, Heliopsis helianthodies, Poa pratensis, and Stipa spartea. Floristic composition, however, varied with respect to management, topography, season of evaluation, and prairie size. Frequent mowing reduced canopy cover of native species, such as Andropogon gerardii, by 21% while increasing that of non-native species, such as Bromus inermis, by 35%. In addition, frequently mowed sites contained a greater number of disturbance species (14 species), such as Conyza canadensis and Setaria glauca, than less frequently mowed sites (6 species). Canopy cover on sites mowed early in the summer averaged 54% higher for warm-season species and 26% lower for cool-season species than on sites mowed in late summer. Total vegetative cover, total grass cover, and total forb cover were lowest for hilltops and south-facing slopes although canopy cover of individual species varied with respect to topographic setting. Total vegetative cover and number of species recorded were highest in August evaluations; canopy cover of individual species varied throughout the growing season. A significant correlation was found between prairie size and Species Richness (P=0.02)
Low-Temperature Excitations of Dilute Lattice Spin Glasses
A new approach to exploring low-temperature excitations in finite-dimensional
lattice spin glasses is proposed. By focusing on bond-diluted lattices just
above the percolation threshold, large system sizes can be obtained which
lead to enhanced scaling regimes and more accurate exponents. Furthermore, this
method in principle remains practical for any dimension, yielding exponents
that so far have been elusive. This approach is demonstrated by determining the
stiffness exponent for dimensions , (the upper critical dimension),
and . Key is the application of an exact reduction algorithm, which
eliminates a large fraction of spins, so that the reduced lattices never exceed
variables for sizes as large as L=30 in , L=9 in , or L=8
in . Finite size scaling analysis gives for ,
significantly improving on previous work. The results for and ,
and , are entirely new and are compared with
mean-field predictions made for d>=6.Comment: 7 pages, LaTex, 7 ps-figures included, added result for stiffness in
d=7, as to appear in Europhysics Letters (see
http://www.physics.emory.edu/faculty/boettcher/ for related information
Estimation of Genetic Parameters for Concentrations of Milk Urea Nitrogen
The objective of this study was to use field data collected by dairy herd improvement programs to estimate genetic parameters for concentrations of milk urea nitrogen (MUN). Edited data were 36,074 test-day records of MUN and yields of milk, fat, and protein obtained from 6102 cows in Holstein herds in Ontario, Canada. Data were divided into three sets, for the first three lactations. Two analyses were performed on data from each lactation. The first procedure used ANOVA to estimate the significance of the effects of several environmental factors on MUN. Herd-test-day effects had the most significant impact on MUN. Effects of stage of lactation were also important, and MUN levels tended to increase from the time of peak yield until the end of lactation. The second analysis used a random regression model to estimate heritabilities and genetic correlations of MUN and the yield traits. Heritability estimates for MUN in lactations one, two, and three were 0.44, 0.59, and 0.48, respectively. Heritabilities for the yield traits were of a similar magnitude. Little relationship was observed between MUN and yield. Raw phenotypic correlations were all <0.10 (absolute value). Genetic correlations with production traits were close to zero in lactations one and three and only slightly positive in lactation two. The results indicate that selection on MUN is possible, but relationships between MUN and other economically important traits such as metabolic disease and fertility are needed
Extremal Optimization for Graph Partitioning
Extremal optimization is a new general-purpose method for approximating
solutions to hard optimization problems. We study the method in detail by way
of the NP-hard graph partitioning problem. We discuss the scaling behavior of
extremal optimization, focusing on the convergence of the average run as a
function of runtime and system size. The method has a single free parameter,
which we determine numerically and justify using a simple argument. Our
numerical results demonstrate that on random graphs, extremal optimization
maintains consistent accuracy for increasing system sizes, with an
approximation error decreasing over runtime roughly as a power law t^(-0.4). On
geometrically structured graphs, the scaling of results from the average run
suggests that these are far from optimal, with large fluctuations between
individual trials. But when only the best runs are considered, results
consistent with theoretical arguments are recovered.Comment: 34 pages, RevTex4, 1 table and 20 ps-figures included, related papers
available at http://www.physics.emory.edu/faculty/boettcher
VHE observations of the gamma-ray binary system LS 5039 with H.E.S.S
LS 5039 is a gamma-ray binary system observed in a broad energy range, from
radio to TeV energies. The binary system exhibits both flux and spectral
modulation as a function of its orbital period. The X-ray and very-high-energy
(VHE, E > 100 GeV) gamma-ray fluxes display a maximum/minimum at
inferior/superior conjunction, with spectra becoming respectively
harder/softer, a behaviour that is completely reversed in the high-energy
domain (HE, 0.1 < E < 100 GeV). The HE spectrum cuts off at a few GeV, with a
new hard component emerging at E > 10 GeV that is compatible with the
low-energy tail of the TeV emission. The low 10 - 100 GeV flux, however, makes
the HE and VHE components difficult to reconcile with a scenario including
emission from only a single particle population. We report on new observations
of LS 5039 conducted with the High Energy Stereoscopic System (H.E.S.S.)
telescopes from 2006 to 2015. This new data set enables for an
unprecedentedly-deep phase-folded coverage of the source at TeV energies, as
well as an extension of the VHE spectral range down to ~120 GeV, which makes LS
5039 the first gamma-ray binary system in which a spectral overlap between
satellite and ground-based gamma-ray observatories is obtained.Comment: Proceeding for ICRC 201
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