1,778 research outputs found
Racing Multi-Objective Selection Probabilities
In the context of Noisy Multi-Objective Optimization, dealing with
uncertainties requires the decision maker to define some preferences about how
to handle them, through some statistics (e.g., mean, median) to be used to
evaluate the qualities of the solutions, and define the corresponding Pareto
set. Approximating these statistics requires repeated samplings of the
population, drastically increasing the overall computational cost. To tackle
this issue, this paper proposes to directly estimate the probability of each
individual to be selected, using some Hoeffding races to dynamically assign the
estimation budget during the selection step. The proposed racing approach is
validated against static budget approaches with NSGA-II on noisy versions of
the ZDT benchmark functions
Exact solitary and periodic-wave solutions of the K(2,2) equation (defocusing branch)
An auxiliary elliptic equation method is presented for constructing exact solitary and periodic travelling-wave solutions of the K(2, 2) equation (defocusing branch). Some known results in the literature are recovered more efficiently, and some new exact travelling-wave solutions are obtained. Also, new stationary-wave solutions are obtained
Reconstructing the Equation of State of Tachyon
Recent progress in theoretical physics suggests that the dark energy in the
universe might be resulted from the rolling tachyon field of string theory.
Measurements to SNe Ia can be helpful to reconstruct the equation of state of
the rolling tachyon which is a possible candidate of dark energy. We present a
numerical analysis for the evolution of the equation of state of the rolling
tachyon and derive the reconstruction equations for the equation of state as
well as the potential.Comment: 6 pages, 3 figures, to appear Phys. Rev.
Attractor Solution of Phantom Field
In light of recent study on the dark energy models that manifest an equation
of state , we investigate the cosmological evolution of phantom field in
a specific potential, exponential potential in this paper. The phase plane
analysis show that the there is a late time attractor solution in this model,
which address the similar issues as that of fine tuning problems in
conventional quintessence models. The equation of state is determined by
the attractor solution which is dependent on the parameter in the
potential. We also show that this model is stable for our present observable
universe.Comment: 9 pages, 3 ps figures; typos corrected, references updated, this is
the final version to match the published versio
Large Scale Structure Formation with Global Topological Defects. A new Formalism and its implementation by numerical simulations
We investigate cosmological structure formation seeded by topological defects
which may form during a phase transition in the early universe. First we derive
a partially new, local and gauge invariant system of perturbation equations to
treat microwave background and dark matter fluctuations induced by topological
defects or any other type of seeds. We then show that this system is well
suited for numerical analysis of structure formation by applying it to seeds
induced by fluctuations of a global scalar field. Our numerical results are
complementary to previous investigations since we use substantially different
methods. The resulting microwave background fluctuations are compatible with
older simulations. We also obtain a scale invariant spectrum of fluctuations
with about the same amplitude. However, our dark matter results yield a smaller
bias parameter compatible with on a scale of in contrast to
previous work which yielded to large bias factors. Our conclusions are thus
more positive. According to the aspects analyzed in this work, global
topological defect induced fluctuations yield viable scenarios of structure
formation and do better than standard CDM on large scales.Comment: uuencoded, compressed tar-file containing the text in LaTeX and 12
Postscript Figures, 41 page
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Microstructure Evolution During Spray Rolling and Heat Treatment of 2124 Al
Spray rolling is a strip casting technology that combines elements of spray forming and twin-roll casting. It consists of atomizing molten metal with a high velocity inert gas, quenching the resultant droplets in flight, and directing the spray between mill rolls. In-flight convection heat transfer from atomized droplets and conduction heat transfer at the rolls rapidly move an alloy’s latent heat. Hot deformation of the semi-solid material in the rolls results in fully consolidated, rapidly-solidified product. While similar in many ways to twin-roll casting, spray rolling is able to process a broader range of alloys and operates at a higher production rate. A laboratory-scale strip caster has been constructed at INL and used to evaluate the interplay of processing parameters and strip quality while producing strips up to 200 mm wide and 1.6 – 6.4 mm thick. Plans are underway to scale to 600 mm width and demonstrate steady-state operation. As-spray-rolled strip is characterized by a flat, uniformly thick profile with minimal porosity or segregation. This paper examines how processing parameters influence the microstructure transformations that take place during spray rolling and post-deposition heat treatment of 2124 Al
217 000-year-old DNA sequences of green sulfur bacteria in Mediterranean sapropels and their implications for the reconstruction of the paleoenvironment
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Society for Applied Microbiology and Blackwell for personal use, not for redistribution. The definitive version was published in Environmental Microbiology 9 (2007): 238–249, doi:10.1111/j.1462-2920.2006.01134.x.Deep-sea sediments of the eastern Mediterranean harbor a series of dark, organic carbon-rich layers, so-called sapropels. Within these layers, the carotenoid isorenieratene was detected. Since
it is specific for the obligately anaerobic phototrophic green sulfur bacteria, the presence of isorenieratene may suggest that extended water column anoxia occurred in the ancient Mediterranean Sea during periods of sapropel formation. Only three carotenoids (isorenieratene, β-isorenieratene and chlorobactene) are typical for green sulfur bacteria and thus do not permit
to differentiate between the ~80 known phylotypes. In order to reconstruct the paleoecological conditions in more detail, we searched for fossil 16S rRNA gene sequences of green sulfur bacteria employing ancient DNA methodology. 540 bp-long fossil sequences could indeed be amplified from up to 217,000-year-old sapropels. In addition, such sequences were also recovered from carbon-lean intermediate sediment layers deposited during times of an entirely
oxic water column. Unexpectedly, however, all the recovered 16S rRNA gene sequences
grouped with freshwater or brackish, rather than truly marine, types of green sulfur bacteria. It is therefore feasible that the molecular remains of green sulfur bacteria originated from populations
which thrived in adjacent freshwater or estuarine coastal environments rather than from an indigenous pelagic population.This work was funded by the Deutsche Forschungsgemeinschaft (grants Ov 20/3-2 and Ov 20/8-1 to 8-3)
A genomic portrait of the emergence, evolution, and global spread of a methicillin-resistant staphylococcus aureus pandemic
The widespread use of antibiotics in association with high-density clinical care has driven the emergence of drug-resistant bacteria that are adapted to thrive in hospitalized patients. Of particular concern are globally disseminated methicillin-resistant Staphylococcus aureus (MRSA) clones that cause outbreaks and epidemics associated with health care. The most rapidly spreading and tenacious health-care-associated clone in Europe currently is EMRSA-15, which was first detected in the UK in the early 1990s and subsequently spread throughout Europe and beyond. Using phylogenomic methods to analyze the genome sequences for 193 S. aureus isolates, we were able to show that the current pandemic population of EMRSA-15 descends from a health-care-associated MRSA epidemic that spread throughout England in the 1980s, which had itself previously emerged from a primarily community-associated methicillin-sensitive population. The emergence of fluoroquinolone resistance in this EMRSA-15 subclone in the English Midlands during the mid-1980s appears to have played a key role in triggering pandemic spread, and occurred shortly after the first clinical trials of this drug. Genome-based coalescence analysis estimated that the population of this subclone over the last 20 yr has grown four times faster than its progenitor. Using comparative genomic analysis we identified the molecular genetic basis of 99.8% of the antimicrobial resistance phenotypes of the isolates, highlighting the potential of pathogen genome sequencing as a diagnostic tool. We document the genetic changes associated with adaptation to the hospital environment and with increasing drug resistance over time, and how MRSA evolution likely has been influenced by country-specific drug use regimens
Validity of numerical trajectories in the synchronization transition of complex systems
We investigate the relationship between the loss of synchronization and the
onset of shadowing breakdown {\it via} unstable dimension variability in
complex systems. In the neighborhood of the critical transition to strongly
non-hyperbolic behavior, the system undergoes on-off intermittency with respect
to the synchronization state. There are potentially severe consequences of
these facts on the validity of the computer-generated trajectories obtained
from dynamical systems whose synchronization manifolds share the same
non-hyperbolic properties.Comment: 4 pages, 4 figure
Exciton swapping in a twisted graphene bilayer as a solid-state realization of a two-brane model
It is shown that exciton swapping between two graphene sheets may occur under
specific conditions. A magnetically tunable optical filter is described to
demonstrate this new effect. Mathematically, it is shown that two turbostratic
graphene layers can be described as a "noncommutative" two-sheeted
(2+1)-spacetime thanks to a formalism previously introduced for the study of
braneworlds in high energy physics. The Hamiltonian of the model contains a
coupling term connecting the two layers which is similar to the coupling
existing between two braneworlds at a quantum level. In the present case, this
term is related to a K-K' intervalley coupling. In addition, the experimental
observation of this effect could be a way to assess the relevance of some
theoretical concepts of the braneworld hypothesis.Comment: 15 pages, 3 figures, final version published in European Physical
Journal
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