1,327 research outputs found
Accelerating random walks by disorder
We investigate the dynamic impact of heterogeneous environments on
superdiffusive random walks known as L\'evy flights. We devote particular
attention to the relative weight of source and target locations on the rates
for spatial displacements of the random walk. Unlike ordinary random walks
which are slowed down for all values of the relative weight of source and
target, non-local superdiffusive processes show distinct regimes of attenuation
and acceleration for increased source and target weight, respectively.
Consequently, spatial inhomogeneities can facilitate the spread of
superdiffusive processes, in contrast to common belief that external disorder
generally slows down stochastic processes. Our results are based on a novel
type of fractional Fokker-Planck equation which we investigate numerically and
by perturbation theory for weak disorder.Comment: 8 pages, 5 figure
Levy flights and Levy -Schroedinger semigroups
We analyze two different confining mechanisms for L\'{e}vy flights in the
presence of external potentials. One of them is due to a conservative force in
the corresponding Langevin equation. Another is implemented by
Levy-Schroedinger semigroups which induce so-called topological Levy processes
(Levy flights with locally modified jump rates in the master equation). Given a
stationary probability function (pdf) associated with the Langevin-based
fractional Fokker-Planck equation, we demonstrate that generically there exists
a topological L\'{e}vy process with the very same invariant pdf and in the
reverse.Comment: To appear in Cent. Eur. J. Phys. (2010
Petroleum exploration subprogram: Geological interpretation of proportional imagery from ERTS-A satellite
There are no author-identified significant results in this report
Video Pandemics: Worldwide Viral Spreading of Psy's Gangnam Style Video
Viral videos can reach global penetration traveling through international
channels of communication similarly to real diseases starting from a
well-localized source. In past centuries, disease fronts propagated in a
concentric spatial fashion from the the source of the outbreak via the short
range human contact network. The emergence of long-distance air-travel changed
these ancient patterns. However, recently, Brockmann and Helbing have shown
that concentric propagation waves can be reinstated if propagation time and
distance is measured in the flight-time and travel volume weighted underlying
air-travel network. Here, we adopt this method for the analysis of viral meme
propagation in Twitter messages, and define a similar weighted network distance
in the communication network connecting countries and states of the World. We
recover a wave-like behavior on average and assess the randomizing effect of
non-locality of spreading. We show that similar result can be recovered from
Google Trends data as well.Comment: 10 page
Genomics reveals a new genus and species from a single female specimen (Lepidoptera: Hesperiidae: Hesperiinae: Hesperiini: Moncina)
New taxa in Hesperiidae (Lepidoptera: Papilionoidea) are traditionally proposed after inspection of male genitalia, which largely form the basis for Hesperiidae taxonomy. However, with genomic DNA seÂquencing, even a single female specimen can be placed in a phylogenetic context of existing classification and taxonomically assigned with confidence. Genomic sequencing of an unusually patterned Hesperiidae female from San Martin, Peru, characterized by pearly spots outlining an inverted heart pattern on the rust-colored ventral hindwing, reveals that it represents an undescribed genus and species named here as Gemmia buechei Brockmann and Grishin, new genus and new species.
Neotropical regions are rich in undescribed species. While many of them may be cryptic and evade recognition by visual inspection (Hebert et al. 2004), others are distinctive and can be recognized as new at the first glance (Turland et al. 2012). However, it may be a challenge to place such distinctive species within a taxonomic hierarÂchy, in particular when only a single specimen is known. Hesperiidae taxonomy relies heavily on the analysis of male genitalia, and descriptions of genus-group taxa traditionally report the structure of male genitalia. However, genera are defined as monophyletic groups of species, and a confident phylogeny that includes all close relatives is a reliable way to define them (Cong et al. 2019; Li et al. 2019; Zhang et al. 2019, 2020, 2022). Here, we illustrate this approach and propose a new Hesperiidae genus based on a single distinctively patterned female specimen. We believe that bringing a new species and the new genus to the attention of researchers has advantages over waiting to find more of its specimens, in particular males. The genomics-based approach that we use puts the phylogenetic placement of this new taxon on a strong footing
Detection of Endoleaks Following Thoracic and Abdominal Aortic Endovascular Aortic Repair—: A Comparison of Standard and Dynamic 4D-Computed Tomography Angiography
Purpose:
Endoleaks are a common complication after endovascular aortic repair (EVAR) and thoracic endovascular aortic repair (TEVAR). The detection and correct classification of endoleaks is essential for the further treatment of affected patients. However, standard computed tomography angiography (CTA) provides no hemodynamic information on endoleaks, which can result in misclassification in complex cases. The aim of this study was to compare standard CTA (sCTA) with dynamic, dual-energy CTA (dCTA) for detection and classification of endoleaks following EVAR or TEVAR.
Materials and Methods:
This retrospective evaluation compared 69 sCTA diagnostic examinations performed on 50 different patients with 89 dCTA diagnostic examinations performed on 69 different patients.
Results:
In total, 15.9% of sCTA examinations and 49.4% of dCTA examinations led to the detection of endoleaks. With sCTA, 20.0% of patients were diagnosed with endoleaks, while with dCTA, 37.7% of patients were diagnosed with endoleaks. With sCTA, mainly Type 1 endoleaks were detected, whereas, with dCTA, the types of detected endoleaks were more evenly distributed. In comparison with the literature, the frequencies of endoleak types detected with dCTA better reflect the natural distribution than the frequencies detected with standard CTA.
Conclusion:
Based on the retrospective comparative evaluation, dCTA could pose a valuable supplementary diagnostic tool resulting in a more accurate and realistic detection and classification of suspected endoleaks
Multiscale mobility networks and the large scale spreading of infectious diseases
Among the realistic ingredients to be considered in the computational
modeling of infectious diseases, human mobility represents a crucial challenge
both on the theoretical side and in view of the limited availability of
empirical data. In order to study the interplay between small-scale commuting
flows and long-range airline traffic in shaping the spatio-temporal pattern of
a global epidemic we i) analyze mobility data from 29 countries around the
world and find a gravity model able to provide a global description of
commuting patterns up to 300 kms; ii) integrate in a worldwide structured
metapopulation epidemic model a time-scale separation technique for evaluating
the force of infection due to multiscale mobility processes in the disease
dynamics. Commuting flows are found, on average, to be one order of magnitude
larger than airline flows. However, their introduction into the worldwide model
shows that the large scale pattern of the simulated epidemic exhibits only
small variations with respect to the baseline case where only airline traffic
is considered. The presence of short range mobility increases however the
synchronization of subpopulations in close proximity and affects the epidemic
behavior at the periphery of the airline transportation infrastructure. The
present approach outlines the possibility for the definition of layered
computational approaches where different modeling assumptions and granularities
can be used consistently in a unifying multi-scale framework.Comment: 10 pages, 4 figures, 1 tabl
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
Neutron stars with isovector scalar correlations
Neutron stars with the isovector scalar -field are studied in the
framework of the relativistic mean field () approach in a pure nucleon
plus lepton scheme. The -field leads to a larger repulsion in dense
neutron-rich matter and to a definite splitting of proton and neutron effective
masses. Both features are influencing the stability conditions of the neutron
stars. Two parametrizations for the effective nonlinear Lagrangian density are
used to calculate the nuclear equation of state () and the neutron star
properties, and compared to correlated Dirac-Brueckner results. We conclude
that in order to reproduce reasonable nuclear structure and neutron star
properties within a approach a density dependence of the coupling
constants is required.Comment: 11 pages, 5 figures, revtex4 styl
Momentum-Dependent Mean Field Based Upon the Dirac-Brueckner Approach for Nuclear Matter
A momentum-dependent mean field potential, suitable for application in the
transport-model description of nucleus-nucleus collisions, is derived in a
microscopic way. The derivation is based upon the Bonn meson-exchange model for
the nucleon-nucleon interaction and the Dirac-Brueckner approach for nuclear
matter. The properties of the microscopic mean field are examined and compared
with phenomenological parametrizations which are commonly used in
transport-model calculations.Comment: 15 pages text (RevTex) and 4 figures (postscript in a separate
uuencoded file), UI-NTH-930
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