67,165 research outputs found
Job-shop scheduling with an adaptive neural network and local search hybrid approach
This article is posted here with permission from IEEE - Copyright @ 2006 IEEEJob-shop scheduling is one of the most difficult production scheduling problems in industry. This paper proposes an adaptive neural network and local search hybrid approach for the job-shop scheduling problem. The adaptive neural network is constructed based on constraint satisfactions of job-shop scheduling and can adapt its structure and neuron connections during the solving process. The neural network is used to solve feasible schedules for the job-shop scheduling problem while the local search scheme aims to improve the performance by searching the neighbourhood of a given feasible schedule. The experimental study validates the proposed hybrid approach for job-shop scheduling regarding the quality of solutions and the computing speed
Cluster Structure in Cosmological Simulations I: Correlation to Observables, Mass Estimates, and Evolution
We use Enzo, a hybrid Eulerian AMR/N-body code including non-gravitational
heating and cooling, to explore the morphology of the X-ray gas in clusters of
galaxies and its evolution in current generation cosmological simulations. We
employ and compare two observationally motivated structure measures: power
ratios and centroid shift. Overall, the structure of our simulated clusters
compares remarkably well to low-redshift observations, although some
differences remain that may point to incomplete gas physics. We find no
dependence on cluster structure in the mass-observable scaling relations, T_X-M
and Y_X-M, when using the true cluster masses. However, estimates of the total
mass based on the assumption of hydrostatic equilibrium, as assumed in
observational studies, are systematically low. We show that the hydrostatic
mass bias strongly correlates with cluster structure and, more weakly, with
cluster mass. When the hydrostatic masses are used, the mass-observable scaling
relations and gas mass fractions depend significantly on cluster morphology,
and the true relations are not recovered even if the most relaxed clusters are
used. We show that cluster structure, via the power ratios, can be used to
effectively correct the hydrostatic mass estimates and mass-scaling relations,
suggesting that we can calibrate for this systematic effect in cosmological
studies. Similar to observational studies, we find that cluster structure,
particularly centroid shift, evolves with redshift. This evolution is mild but
will lead to additional errors at high redshift. Projection along the line of
sight leads to significant uncertainty in the structure of individual clusters:
less than 50% of clusters which appear relaxed in projection based on our
structure measures are truly relaxed.Comment: 57 pages, 18 figures, accepted to ApJ, updated definition of T_X and
M_gas but results unchanged, for version with full resolution figures, see
http://www.ociw.edu/~tesla/sims.ps.g
Compensatory evolution and the origins of innovations
Cryptic genetic sequences have attenuated effects on phenotypes. In the
classic view, relaxed selection allows cryptic genetic diversity to build up
across individuals in a population, providing alleles that may later contribute
to adaptation when co-opted - e.g. following a mutation increasing expression
from a low, attenuated baseline. This view is described, for example, by the
metaphor of the spread of a population across a neutral network in genotype
space. As an alternative view, consider the fact that most phenotypic traits
are affected by multiple sequences, including cryptic ones. Even in a strictly
clonal population, the co-option of cryptic sequences at different loci may
have different phenotypic effects and offer the population multiple adaptive
possibilities. Here, we model the evolution of quantitative phenotypic
characters encoded by cryptic sequences, and compare the relative contributions
of genetic diversity and of variation across sites to the phenotypic potential
of a population. We show that most of the phenotypic variation accessible
through co-option would exist even in populations with no polymorphism. This is
made possible by a history of compensatory evolution, whereby the phenotypic
effect of a cryptic mutation at one site was balanced by mutations elsewhere in
the genome, leading to a diversity of cryptic effect sizes across sites rather
than across individuals. Cryptic sequences might accelerate adaptation and
facilitate large phenotypic changes even in the absence of genetic diversity,
as traditionally defined in terms of alternative alleles
Characterizing Galaxy Clusters with Gravitational Potential
We propose a simple estimator for the gravitational potential of cluster-size
halos using the temperature and density profiles of the intracluster gas based
on the assumptions of hydrostatic equilibrium and spherical symmetry. Using
high resolution cosmological simulations of galaxy clusters, we show that the
scaling relation between this estimator and the gravitational potential has a
small intrinsic scatter of ~8%-15%, and it is insensitive to baryon physics
outside the cluster core. The slope and the normalization of the scaling
relation vary weakly with redshift, and they are relatively independent of the
choice of radial range used and the dynamical states of the clusters. The
results presented here provide a possible way for using the cluster potential
function as an alternative to the cluster mass function in constraining
cosmology using galaxy clusters.Comment: 10 pages, 7 figures, 4 tables. Matching the version accepted for
publication in the Astrophysical Journa
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