3,664 research outputs found
Effects of Preference for Attachment to Low-degree Nodes on the Degree Distributions of a Growing Directed Network and a Simple Food-Web Model
We study the growth of a directed network, in which the growth is constrained
by the cost of adding links to the existing nodes. We propose a new
preferential-attachment scheme, in which a new node attaches to an existing
node i with probability proportional to 1/k_i, where k_i is the number of
outgoing links at i. We calculate the degree distribution for the outgoing
links in the asymptotic regime (t->infinity), both analytically and by Monte
Carlo simulations. The distribution decays like k c^k/Gamma(k) for large k,
where c is a constant. We investigate the effect of this
preferential-attachment scheme, by comparing the results to an equivalent
growth model with a degree-independent probability of attachment, which gives
an exponential outdegree distribution. Also, we relate this mechanism to simple
food-web models by implementing it in the cascade model. We show that the
low-degree preferential-attachment mechanism breaks the symmetry between in-
and outdegree distributions in the cascade model. It also causes a faster decay
in the tails of the outdegree distributions for both our network growth model
and the cascade model.Comment: 10 pages, 7 figures. A new figure added. Minor modifications made in
the tex
The "zeroth law" of turbulence: Isotropic turbulence simulations revisited
The dimensionless kinetic energy dissipation rate C_epsilon is estimated from
numerical simulations of statistically stationary isotropic box turbulence that
is slightly compressible. The Taylor microscale Reynolds number Re_lambda range
is 20 < Re_lambda < 220 and the statistical stationarity is achieved with a
random phase forcing method. The strong Re_lambda dependence of C_epsilon
abates when Re_lambda approx. 100 after which C_epsilon slowly approaches
approx 0.5 a value slightly different to previously reported simulations but in
good agreement with experimental results. If C_epsilon is estimated at a
specific time step from the time series of the quantities involved it is
necessary to account for the time lag between energy injection and energy
dissipation. Also, the resulting value can differ from the ensemble averaged
value by up to +-30%. This may explain the spread in results from previously
published estimates of C_epsilon.Comment: 7 pages, 7 figures. Submitted to Phys. Rev.
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
Weak Gravitational Lensing by a Sample of X-Ray Luminous Clusters of Galaxies -- II. Comparison with Virial Masses
Dynamic velocity dispersion and mass estimates are given for a sample of five
X-ray luminous rich clusters of galaxies at intermediate redshifts (z~0.3)
drawn from a sample of 39 clusters for which we have obtained gravitational
lens mass estimates. The velocity dispersions are determined from between 9 and
20 redshifts measured with the LDSS spectrograph of the William Herschel
Telescope, and virial radii are determined from imaging using the UH8K mosaic
CCD camera on the University of Hawaii 2.24m telescope.
Including clusters with velocity dispersions taken from the literature, we
have velocity dispersion estimates for 12 clusters in our gravitational lensing
sample. For this sample we compare the dynamical velocity dispersion estimates
with our estimates of the velocity dispersions made from gravitational lensing
by fitting a singular isothermal sphere profile to the observed tangential weak
lensing distortion as a function of radius. In all but two clusters, we find a
good agreement between the velocity dispersion estimates based on spectroscopy
and on weak lensing.Comment: 9 pages, 4 figures, accepted for publication in ApJ. Version in
emulateapj format with only minor change
Periodic orbit quantization of the Sinai billiard in the small scatterer limit
We consider the semiclassical quantization of the Sinai billiard for disk
radii R small compared to the wave length 2 pi/k. Via the application of the
periodic orbit theory of diffraction we derive the semiclassical spectral
determinant. The limitations of the derived determinant are studied by
comparing it to the exact KKR determinant, which we generalize here for the A_1
subspace. With the help of the Ewald resummation method developed for the full
KKR determinant we transfer the complex diffractive determinant to a real form.
The real zeros of the determinant are the quantum eigenvalues in semiclassical
approximation. The essential parameter is the strength of the scatterer
c=J_0(kR)/Y_0(kR). Surprisingly, this can take any value between plus and minus
infinity within the range of validity of the diffractive approximation kR <<4.
We study the statistics exhibited by spectra for fixed values of c. It is
Poissonian for |c|=infinity, provided the disk is placed inside a rectangle
whose sides obeys some constraints. For c=0 we find a good agreement of the
level spacing distribution with GOE, whereas the form factor and two-point
correlation function are similar but exhibit larger deviations. By varying the
parameter c from 0 to infinity the level statistics interpolates smoothly
between these limiting cases.Comment: 17 pages LaTeX, 5 postscript figures, submitted to J. Phys. A: Math.
Ge
Putting evolutionary biology back in the ecological theatre: a demographic framework mapping genes to communities
Question: How can we link genotypic, phenotypic, individual, population, and community levels of organization so as to illuminate general ecological and evolutionary processes and provide a framework for a quantitative, integrative evolutionary biology? Framework: We introduce an evolutionary framework that maps different levels of biological diversity onto one another. We provide (1) an overview of maps linking levels of biological organization and (2) a guideline of how to analyse the complexity of relationships from genes to population growth. Method: We specify the appropriate levels of biological organization for responses to selection, for opportunities for selection, and for selection itself. We map between them and embed these maps into an ecological setting
Teriparatide improves early callus formation in distal radial fractures: Analysis of a subgroup of patients within a randomized trial
Background Teriparatide (parathyreoid hormone; PTH 1-34) increases skeletal mass in humans and improves fracture healing in animals. A recent randomized multicenter trial of nonoperated distal radial fractures showed a moderate shortening of the time to restoration of cortical continuity by treatment with 20 ÎĽg (low-dose) teriparatide per day, but not with 40 ÎĽg (high-dose). As radiographic cortical continuity appears late in the healing process, perhaps too late for clinical relevance, we studied the qualitative appearance of the callus 5 weeks after fracture
Theory of the Eigler-swith
We suggest a simple model to describe the reversible field-induced transfer
of a single Xe-atom in a scanning tunneling microscope, --- the Eigler-switch.
The inelasticly tunneling electrons give rise to fluctuating forces on and
damping of the Xe-atom resulting in an effective current dependent temperature.
The rate of transfer is controlled by the well-known Arrhenius law with this
effective temperature. The directionality of atom transfer is discussed, and
the importance of use of non-equlibrium-formalism for the electronic
environment is emphasized. The theory constitutes a formal derivation and
generalization of the so-called Desorption Induced by Multiple Electron
Transitions (DIMET) point of view.Comment: 13 pages (including 2 figures in separate LaTeX-files with
ps-\specials), REVTEX 3.
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