206 research outputs found
A methodology for producing reliable software, volume 1
An investigation into the areas having an impact on producing reliable software including automated verification tools, software modeling, testing techniques, structured programming, and management techniques is presented. This final report contains the results of this investigation, analysis of each technique, and the definition of a methodology for producing reliable software
A coalescence model for freely decaying two-dimensional turbulence
We propose a ballistic coalescence model (punctuated-Hamiltonian approach)
mimicking the fusion of vortices in freely decaying two-dimensional turbulence.
A temporal scaling behaviour is reached where the vortex density evolves like
. A mean-field analytical argument yielding the approximation
is shown to slightly overestimate the decay exponent whereas
Molecular Dynamics simulations give , in agreement with
recent laboratory experiments and simulations of Navier-Stokes equation.Comment: 6 pages, 1 figure, to appear in Europhysics Letter
Optimal Traffic Networks
Inspired by studies on the airports' network and the physical Internet, we
propose a general model of weighted networks via an optimization principle. The
topology of the optimal network turns out to be a spanning tree that minimizes
a combination of topological and metric quantities. It is characterized by a
strongly heterogeneous traffic, non-trivial correlations between distance and
traffic and a broadly distributed centrality. A clear spatial hierarchical
organization, with local hubs distributing traffic in smaller regions, emerges
as a result of the optimization. Varying the parameters of the cost function,
different classes of trees are recovered, including in particular the minimum
spanning tree and the shortest path tree. These results suggest that a
variational approach represents an alternative and possibly very meaningful
path to the study of the structure of complex weighted networks.Comment: 4 pages, 4 figures, final revised versio
A review of issues and challenges
Determining the ages of young planetary surfaces relies on using populations
of small, often sub-km diameter impact craters due to the higher frequency at
which they form. Smaller craters however can be less reliable for estimating
ages as their size-frequency distribution is more susceptible to alteration
with debate as to whether they should be used at all. With the current
plethora of meter-scale resolution images acquired of the lunar and Martian
surfaces, small craters have been widely used to derive model ages to
establish the temporal relation of recent geologic events. In this review
paper, we discuss the many factors that make smaller craters particularly
challenging to use and should be taken into consideration when crater counts
are confined to small crater diameters. Establishing confidence in a model age
ultimately requires an understanding of the geologic context of the surface
being dated as reliability can vary considerably and limitations of the dating
technique should be considered in applying ages to any geologic
interpretation
Searching for Saturn's Dust Swarm: Limits on the size distribution of Irregular Satellites from km to micron sizes
We describe a search for dust created in collisions between the Saturnian
irregular satellites using archival \emph{Spitzer} MIPS observations. Although
we detected a degree scale Saturn-centric excess that might be attributed to an
irregular satellite dust cloud, we attribute it to the far-field wings of the
PSF due to nearby Saturn. The Spitzer PSF is poorly characterised at such
radial distances, and we expect PSF characterisation to be the main issue for
future observations that aim to detect such dust. The observations place an
upper limit on the level of dust in the outer reaches of the Saturnian system,
and constrain how the size distribution extrapolates from the smallest known
(few km) size irregulars down to micron-size dust. Because the size
distribution is indicative of the strength properties of irregulars, we show
how our derived upper limit implies irregular satellite strengths more akin to
comets than asteroids. This conclusion is consistent with their presumed
capture from the outer regions of the Solar System.Comment: accepted to MNRA
Tops and Writhing DNA
The torsional elasticity of semiflexible polymers like DNA is of biological
significance. A mathematical treatment of this problem was begun by Fuller
using the relation between link, twist and writhe, but progress has been
hindered by the non-local nature of the writhe. This stands in the way of an
analytic statistical mechanical treatment, which takes into account thermal
fluctuations, in computing the partition function. In this paper we use the
well known analogy with the dynamics of tops to show that when subjected to
stretch and twist, the polymer configurations which dominate the partition
function admit a local writhe formulation in the spirit of Fuller and thus
provide an underlying justification for the use of Fuller's "local writhe
expression" which leads to considerable mathematical simplification in solving
theoretical models of DNA and elucidating their predictions. Our result
facilitates comparison of the theoretical models with single molecule
micromanipulation experiments and computer simulations.Comment: 17 pages two figure
Calculations of the A_1 phonon frequency in photoexcited Tellurium
Calculations of the A_1 phonon frequency in photoexcited tellurium are
presented. The phonon frequency as a function of photoexcited carrier density
and phonon amplitude is determined. Recent pump probe experiments are
interpreted in the light of these calculatons. It is proposed that, in
conjunction with measurements of the phonon period in ultra-fast pump-probe
reflectivity experiments, the calculated frequency shifts can be used to infer
the evolution of the density of photoexcited carriers on a sub-picosecond
time-scale.Comment: 15 pages Latex, 3 postscript figure
PCR reveals significantly higher rates of Trypanosoma cruzi infection than microscopy in the Chagas vector, Triatoma infestans: High rates found in Chuquisaca, Bolivia
<p>Abstract</p> <p>Background</p> <p>The Andean valleys of Bolivia are the only reported location of sylvatic <it>Triatoma infestans</it>, the main vector of Chagas disease in this country, and the high human prevalence of <it>Trypanosoma cruzi </it>infection in this region is hypothesized to result from the ability of vectors to persist in domestic, peri-domestic, and sylvatic environments. Determination of the rate of <it>Trypanosoma </it>infection in its triatomine vectors is an important element in programs directed at reducing human infections. Traditionally, <it>T. cruzi </it>has been detected in insect vectors by direct microscopic examination of extruded feces, or dissection and analysis of the entire bug. Although this technique has proven to be useful, several drawbacks related to its sensitivity especially in the case of small instars and applicability to large numbers of insects and dead specimens have motivated researchers to look for a molecular assay based on the polymerase chain reaction (PCR) as an alternative for parasitic detection of <it>T. cruzi </it>infection in vectors. In the work presented here, we have compared a PCR assay and direct microscopic observation for diagnosis of <it>T. cruzi </it>infection in <it>T. infestans </it>collected in the field from five localities and four habitats in Chuquisaca, Bolivia. The efficacy of the methods was compared across nymphal stages, localities and habitats.</p> <p>Methods</p> <p>We examined 152 nymph and adult <it>T. infestans </it>collected from rural areas in the department of Chuquisaca, Bolivia. For microscopic observation, a few drops of rectal content obtained by abdominal extrusion were diluted with saline solution and compressed between a slide and a cover slip. The presence of motile parasites in 50 microscopic fields was registered using 400× magnification. For the molecular analysis, dissection of the posterior part of the abdomen of each insect followed by DNA extraction and PCR amplification was performed using the TCZ1 (5' – CGA GCT CTT GCC CAC ACG GGT GCT – 3') and TCZ2 (5' – CCT CCA AGC AGC GGA TAG TTC AGG – 3') primers. Amplicons were chromatographed on a 2% agarose gel with a 100 bp size standard, stained with ethidium bromide and viewed with UV fluorescence.</p> <p>For both the microscopy and PCR assays, we calculated sensitivity (number of positives by a method divided by the number of positives by either method) and discrepancy (one method was negative and the other was positive) at the locality, life stage and habitat level. The degree of agreement between PCR and microscopy was determined by calculating Kappa (<it>k</it>) values with 95% confidence intervals.</p> <p>Results</p> <p>We observed a high prevalence of <it>T. cruzi </it>infection in <it>T. infestans </it>(81.16% by PCR and 56.52% by microscopy) and discovered that PCR is significantly more sensitive than microscopic observation. The overall degree of agreement between the two methods was moderate (Kappa = 0.43 ± 0.07). The level of infection is significantly different among communities; however, prevalence was similar among habitats and life stages.</p> <p>Conclusion</p> <p>PCR was significantly more sensitive than microscopy in all habitats, developmental stages and localities in Chuquisaca, Bolivia. Overall we observed a high prevalence of <it>T. cruzi </it>infection in <it>T. infestans </it>in this area of Bolivia; however, microscopy underestimated infection at all levels examined.</p
Gene targeting in adult rhesus macaque fibroblasts
<p>Abstract</p> <p>Background</p> <p>Gene targeting in nonhuman primates has the potential to produce critical animal models for translational studies related to human diseases. Successful gene targeting in fibroblasts followed by somatic cell nuclear transfer (SCNT) has been achieved in several species of large mammals but not yet in primates. Our goal was to establish the protocols necessary to achieve gene targeting in primary culture of adult rhesus macaque fibroblasts as a first step in creating nonhuman primate models of genetic disease using nuclear transfer technology.</p> <p>Results</p> <p>A primary culture of adult male fibroblasts was transfected with hTERT to overcome senescence and allow long term <it>in vitro </it>manipulations. Successful gene targeting of the HPRT locus in rhesus macaques was achieved by electroporating S-phase synchronized cells with a construct containing a SV40 enhancer.</p> <p>Conclusion</p> <p>The cell lines reported here could be used for the production of null mutant rhesus macaque models of human genetic disease using SCNT technology. In addition, given the close evolutionary relationship and biological similarity between rhesus macaques and humans, the protocols described here may prove useful in the genetic engineering of human somatic cells.</p
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