940 research outputs found
Halo phenomenon in finite many-fermion systems. Atom-positron complexes and large-scale study of atomic nuclei
The analysis method proposed in Ref. \cite{rotival07a} is applied to
characterize halo properties in finite many-fermion systems. First, the
versatility of the method is highlighted by applying it to light and
medium-mass nuclei as well as to atom-positron and ion-positronium complexes.
Second, the dependence of nuclear halo properties on the characteristics of the
energy density functional used in self-consistent Hartree-Fock-Bogoliubov
calculations is studied. It is found that (a) the low-density behavior of the
pairing functional and the regularization/renormalization scheme must be chosen
coherently and with care to provide meaningful predictions, (b) the impact of
pairing correlations on halo properties is significant and is the result of two
competing effects, (c) the detailed characteristics of the pairing functional
has however only little importance, (d) halo properties depend significantly on
any ingredient of the energy density functional that influences the location of
single-particle levels; i.e. the effective mass, the tensor terms and the
saturation density of nuclear matter. The latter dependencies give insights to
how experimental data on medium-mass drip-line nuclei can be used in the
distant future to constrain some characteristics of the nuclear energy density
functional. Last but not least, large scale predictions of halos among all
spherical even-even nuclei are performed using specific sets of particle-hole
and particle-particle energy functionals. It is shown that halos in the ground
state of medium-mass nuclei will only be found at the very limit of neutron
stability and for a limited number of elements.Comment: 24 Pages, 32 Figures. Accepted for publication in Phys. Rev. C
back-to back with first part (nucl-th/0702050
Directional persistence & the optimality of run-and-tumble chemotaxis
E. coli does chemotaxis by performing a biased random walk composed of alternating periods of swimming (runs) and reorientations (tumbles). Tumbles are typically modelled as complete directional randomisations but it is known that in wild type E. coli, successive run directions are actually weakly correlated, with a mean directional difference of not, vert, similar63°. We recently presented a model of the evolution of chemotactic swimming strategies in bacteria which is able to quantitatively reproduce the emergence of this correlation. The agreement between model and experiments suggests that directional persistence may serve some function, a hypothesis supported by the results of an earlier model. Here we investigate the effect of persistence on chemotactic efficiency, using a spatial Monte Carlo model of bacterial swimming in a gradient, combined with simulations of natural selection based on chemotactic efficiency. A direct search of the parameter space reveals two attractant gradient regimes, (a) a low-gradient regime, in which efficiency is unaffected by directional persistence and (b) a high-gradient regime, in which persistence can improve chemotactic efficiency. The value of the persistence parameter that maximises this effect corresponds very closely with the value observed experimentally. This result is matched by independent simulations of the evolution of directional memory in a population of model bacteria, which also predict the emergence of persistence in high-gradient conditions. The relationship between optimality and persistence in different environments may reflect a universal property of random-walk foraging algorithms, which must strike a compromise between two competing aims: exploration and exploitation. We also present a new graphical way to generally illustrate the evolution of a particular trait in a population, in terms of variations in an evolvable parameter
Coordinate-space solution of the Skyrme-Hartree-Fock-Bogolyubov equations within spherical symmetry. The program HFBRAD (v1.0)
We describe the first version (v1.00) of the code HFBRAD which solves the
Skyrme-Hartree-Fock or Skyrme-Hartree-Fock-Bogolyubov equations in the
coordinate representation within the spherical symmetry. A realistic
representation of the quasiparticle wave functions on the space lattice allows
for performing calculations up to the particle drip lines. Zero-range
density-dependent interactions are used in the pairing channel. The pairing
energy is calculated by either using a cut-off energy in the quasiparticle
spectrum or the regularization scheme proposed by A. Bulgac and Y. Yu.Comment: 39 pages, 9 figure
Lagrangian Reachabililty
We introduce LRT, a new Lagrangian-based ReachTube computation algorithm that
conservatively approximates the set of reachable states of a nonlinear
dynamical system. LRT makes use of the Cauchy-Green stretching factor (SF),
which is derived from an over-approximation of the gradient of the solution
flows. The SF measures the discrepancy between two states propagated by the
system solution from two initial states lying in a well-defined region, thereby
allowing LRT to compute a reachtube with a ball-overestimate in a metric where
the computed enclosure is as tight as possible. To evaluate its performance, we
implemented a prototype of LRT in C++/Matlab, and ran it on a set of
well-established benchmarks. Our results show that LRT compares very favorably
with respect to the CAPD and Flow* tools.Comment: Accepted to CAV 201
The moderating effect of brand orientation on inter-firm market orientation and performance
While prior research has shown that market and brand orientation
are key contributors to successful business performance, research to
date has not fully explored how inter firm collaboration for these two
key orientations can enhance business performance. The purpose of
the paper is to investigate the relationship between inter-firm market
and performance; to test for the moderating role of brand orientation
in that relationship. A total of 169 completed pairs of surveys were
collected of small and medium enterprises operating internationally
in a variety of industries in Switzerland. The results show that inter-firm
market and brand orientation are two antecedents of marketing and
financial performance. The impact of inter-firm market on marketing
and financial performance is significant when the brand orientation
is favorable. This study extends previous research by examining the
moderating role of brand orientation on inter firm market orientation,
which is important, especially for firms wanting to increase their brand
reputation by entering into partnerships with other firms. Further
research is indicated, to identify the key moderators of the driving
force of inter-firm market in relation to business performance and
the reason why maintaining a strong brand presence is important in
the international marketplace
Domain wall interacting with a black hole: A new example of critical phenomena
We study a simple system that comprises all main features of critical
gravitational collapse, originally discovered by Choptuik and discussed in many
subsequent publications. These features include universality of phenomena,
mass-scaling relations, self-similarity, symmetry between super-critical and
sub-critical solutions, etc.
The system we consider is a stationary membrane (representing a domain wall)
in a static gravitational field of a black hole. For a membrane that spreads to
infinity, the induced 2+1 geometry is asymptotically flat. Besides solutions
with Minkowski topology there exists also solutions with the induced metric and
topology of a 2+1 dimensional black hole. By changing boundary conditions at
infinity, one finds that there is a transition between these two families. This
transition is critical and it possesses all the above-mentioned properties of
critical gravitational collapse. It is remarkable that characteristics of this
transition can be obtained analytically. In particular, we find exact
analytical expressions for scaling exponents and wiggle-periods.
Our results imply that black hole formation as a critical phenomenon is far
more general than one might expect.Comment: 23 pages, 5 postscript figures include
Candidate Genes for Chromosomes 6 and 10: Quantitative Trait Loci for Age-Related Retinal Degeneration in Mice
Purpose: In a previous study, several quantitative trait loci (QTL) that influence age-related degeneration (ageRD) were identified in a cross between the albino strains B6(Cg)-Tyr(c-2J)/J (B6a) and BALB/cByJ (C). The Chromosome (Chr) 6 and Chr 10 QTL were the strongest and most highly significant loci and both involved B6a protective alleles. The QTL were responsible for 21% and 9% of the variance in phenotypes, respectively. We focused on these two QTL to identify candidate genes. Methods: DNA microarrays were used for the two mouse strains at four and eight months of age to identify genes that are differentially regulated and map to either QTL. Gene Ontology (GO) analysis of the differentially expressed genes was performed to identify possible processes and pathways associated with ageRD. To identify additional candidates, database analyses (Positional Medline or PosMed) were used. Based on differential expression, PosMed, and the presence of reported polymorphisms, five genes per QTL were selected for further study by sequencing analysis and qRT-PCR. Tumor necrosis factor, alpha-induced protein 3 (Tnfaip3; on a C57BL/6J (B6) background) was phenotypically tested. Single nucleotide polymorphisms (SNPs) flanking this gene were correlated with outer nuclear layer thickness (ONL), and eight-month-old Tnfaip3(+/-) mice were tested for ageRD. Results: Polymorphisms were found in the coding regions of eight genes. Changes in gene expression were identified by qRT-PCR for Hexokinase 2 (Hk2) and Docking protein 1 (Dok1) at four months and for Dok1 and Tnfaip3 at eight months. Tnfaip3 was selected for phenotypic testing due to differential expression and the presence of two nonsynonymous mutations. However, when ONL thickness was compared in eight-month-old congenic Tnfaip3(+/-) and Tnfaip3(+/+) mice, no differences were found, suggesting that Tnfaip3 is not the quantitative trait gene (QTG) for the Chr 10 QTL. The GO analysis revealed that GO terms associated with stress and cell remodeling are overrepresented in the ageRD-sensitive C strain compared with the B6a strain with age (eight months). In the ageRD-resistant B6a strain, compared with the C strain, GO terms associated with antioxidant response and the regulation of blood vessel size are overrepresented with age. Conclusions: The analyses of differentially expressed genes and the PosMed database yielded candidate genes for the Chr 6 and Chr 10 QTL. HtrA serine peptidase 2 (Htra2), Dok1, and Tnfaip3 were deemed most promising because of their known roles in apoptosis and our finding of nonsynonymous substitutions between B6a and C strains. While Tnfaip3 was excluded as the QTG for the Chr 10 QTL, Dok1 and Htra2 remain good candidates for the Chr 6 QTL. Finally, the GO term analysis further supports the general hypothesis that oxidative stress is involved in ageRD
Adhesion between elastic solids with randomly rough surfaces: comparison of analytical theory with molecular dynamics simulations
The adhesive contact between elastic solids with randomly rough, self affine
fractal surfaces is studied by molecular dynamics (MD) simulations. The
interfacial binding energy obtained from the simulations of nominally flat and
curved surfaces is compared with the predictions of the contact mechanics
theory by Persson. Theoretical and simulation results agree rather well, and
most of the differences observed can be attributed to finite size effects and
to the long-range nature of the interaction between the atoms in the block and
the substrate in the MD model, as compared to the analytical theory which is
for an infinite system with interfacial contact interaction. For curved
surfaces (JKR-type of problem) the effective interfacial energy exhibit a weak
hysteresis which may be due to the influence of local irreversible detachment
processes in the vicinity of the opening crack tip during pull-off.Comment: 6 pages, 6 figure
On a q-extension of Mehta's eigenvectors of the finite Fourier transform for q a root of unity
It is shown that the continuous q-Hermite polynomials for q a root of unity
have simple transformation properties with respect to the classical Fourier
transform. This result is then used to construct q-extended eigenvectors of the
finite Fourier transform in terms of these polynomials.Comment: 12 pages, thoroughly rewritten, the q-extended eigenvectors now
N-periodic with q an M-th root of
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Overview of mathematical approaches used to model bacterial chemotaxis II: bacterial populations
We review the application of mathematical modeling to understanding the behavior of populations of chemotactic bacteria. The application of continuum mathematical models, in particular generalized Keller–Segel models, is discussed along with attempts to incorporate the microscale (individual) behavior on the macroscale, modeling the interaction between different species of bacteria, the interaction of bacteria with their environment, and methods used to obtain experimentally verified parameter values. We allude briefly to the role of modeling pattern formation in understanding collective behavior within bacterial populations. Various aspects of each model are discussed and areas for possible future research are postulated
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