2,342 research outputs found
Photometric variability of candidate white dwarf binary systems from Palomar Transient Factory archival data
We present a sample of 59 periodic variables from the Palomar Transient
Factory, selected from published catalogues of white dwarf (WD) candidates. The
variability can likely be attributed to ellipsoidal variation of the tidally
distorted companion induced by the gravity of the primary (WD or hot subdwarf)
or to the reflection of hot emission by a cooler companion. We searched 11311
spectroscopically or photometrically selected WD candidates from three hot
star/WD catalogues, using the Lomb-Scargle periodogram to single out promising
sources. We present period estimates for the candidates, 45 of which were not
previously identified as periodic variables, and find that most have a period
shorter than a few days. Additionally, we discuss the eclipsing systems in our
sample and present spectroscopic data on selected sources
Finite Density QCD in the Chiral Limit
We present the first results of an exact simulation of full QCD at finite
density in the chiral limit. We have used a MFA (Microcanonical Fermionic
Average) inspired approach for the reconstruction of the Grand Canonical
Partition Function of the theory; using the fugacity expansion of the fermionic
determinant we are able to move continuously in the () plane with
.Comment: 3 pages, LaTeX, 3 figures, uses espcrc2.sty, psfig. Talk presented by
A. Galante at Lattice 97. Correction of some reference
Host carbon sources modulate cell wall architecture, drug resistance and virulence in a fungal pathogen
The survival of all microbes depends upon their ability to respond to environmental challenges. To establish infection, pathogens such as Candida albicans must mount effective stress responses to counter host defences while adapting to dynamic changes in nutrient status within host niches. Studies of C.âalbicans stress adaptation have generally been performed on glucose-grown cells, leaving the effects of alternative carbon sources upon stress resistance largely unexplored. We have shown that growth on alternative carbon sources, such as lactate, strongly influence the resistance of C.âalbicans to antifungal drugs, osmotic and cell wall stresses. Similar trends were observed in clinical isolates and other pathogenic Candida species. The increased stress resistance of C.âalbicans was not dependent on key stress (Hog1) and cell integrity (Mkc1) signalling pathways. Instead, increased stress resistance was promoted by major changes in the architecture and biophysical properties of the cell wall. Glucose- and lactate-grown cells displayed significant differences in cell wall mass, ultrastructure, elasticity and adhesion. Changes in carbon source also altered the virulence of C.âalbicans in models of systemic candidiasis and vaginitis, confirming the importance of alternative carbon sources within host niches during C.âalbicans infection
Lessons from the Congested Clique Applied to MapReduce
The main results of this paper are (I) a simulation algorithm which, under
quite general constraints, transforms algorithms running on the Congested
Clique into algorithms running in the MapReduce model, and (II) a distributed
-coloring algorithm running on the Congested Clique which has an
expected running time of (i) rounds, if ;
and (ii) rounds otherwise. Applying the simulation theorem to
the Congested-Clique -coloring algorithm yields an -round
-coloring algorithm in the MapReduce model.
Our simulation algorithm illustrates a natural correspondence between
per-node bandwidth in the Congested Clique model and memory per machine in the
MapReduce model. In the Congested Clique (and more generally, any network in
the model), the major impediment to constructing fast
algorithms is the restriction on message sizes. Similarly, in the
MapReduce model, the combined restrictions on memory per machine and total
system memory have a dominant effect on algorithm design. In showing a fairly
general simulation algorithm, we highlight the similarities and differences
between these models.Comment: 15 page
SSR markers reveal diversity in Guinea yam (Dioscorea cayenensis/D. rotundata) core set
The genetic diversity of 219 accessions of Guinea yam germplasm from Benin, Congo, CĂŽte dâ Ivoire, Equatorial Guinea, Gabon, Ghana, Nigeria, Sierra Leone and Togo was accessed using 15 microsatelliteloci. High diversity of 0.677 was found among the accessions. An allelic average of 8.06 and polymorphic information content (PIC) value of 0.65 was observed for the markers. The observed heterozygosity value of 0.563 suggests that spontaneous hybridization must have contributed to the ancestry of some of the accessions and improvement by farmers must have been far more often by selection of somatic mutants. The twenty distinct cluster groups generated by the radial phylogram shows that Dioscorea cayenensis and D. rotundata are distinct species with intermediate hybrid forms. There was no relationship between relatedness of the accessions and their geographical area of origin. This study contributes to an increased understanding of the genetic organisation of the coregermplasm
Global phylogeography and evolution of chelonid fibropapilloma-associated herpesvirus
A global phylogeny for chelonid fibropapilloma-associated herpesvirus (CFPHV), the most likely aetiological agent of fibropapillomatosis (FP) in sea turtles, was inferred, using dated sequences, through Bayesian Markov chain Monte Carlo analysis and used to estimate the virus evolutionary rate independent of the evolution of the host, and to resolve the phylogenetic positions of new haplotypes from Puerto Rico and the Gulf of Guinea. Four phylogeographical groups were identified: eastern Pacific, western Atlantic/eastern Caribbean, mid-west Pacific and Atlantic. The latter comprises the Gulf of Guinea and Puerto Rico, suggesting recent virus gene flow between these two regions. One virus haplotype from Florida remained elusive, representing either an independent lineage sharing a common ancestor with all other identified virus variants or an Atlantic representative of the lineage giving rise to the eastern Pacific group. The virus evolutionary rate ranged from 1.62x10(-4) to 2.22x10(-4) substitutions per site per year, which is much faster than what is expected for a herpesvirus. The mean time for the most recent common ancestor of the modern virus variants was estimated at 192.90-429.71 years ago, which, although more recent than previous estimates, still supports an interpretation that the global FP pandemic is not the result of a recent acquisition of a virulence mutation(s). The phylogeographical pattern obtained seems partially to reflect sea turtle movements, whereas altered environments appear to be implicated in current FP outbreaks and in the modern evolutionary history of CFPHV.DNER-PR; US NMFS (NMFS-NOAA) [NA08NMF4720436]; US-Fish and Wildlife Service (USFWS); Sociedad Chelonia; WIDECAST; US Environmental Protection Agency (US-EPA); Lisbon Oceanarium, Portugal; Interdisciplinary Research Center for Animal Health of the Faculty of Veterinary Medicine of the Technical University of Lisbon (FMV/TUL)info:eu-repo/semantics/publishedVersio
Hypergraph Markov Operators, Eigenvalues and Approximation Algorithms
The celebrated Cheeger's Inequality \cite{am85,a86} establishes a bound on
the expansion of a graph via its spectrum. This inequality is central to a rich
spectral theory of graphs, based on studying the eigenvalues and eigenvectors
of the adjacency matrix (and other related matrices) of graphs. It has remained
open to define a suitable spectral model for hypergraphs whose spectra can be
used to estimate various combinatorial properties of the hypergraph.
In this paper we introduce a new hypergraph Laplacian operator (generalizing
the Laplacian matrix of graphs)and study its spectra. We prove a Cheeger-type
inequality for hypergraphs, relating the second smallest eigenvalue of this
operator to the expansion of the hypergraph. We bound other hypergraph
expansion parameters via higher eigenvalues of this operator. We give bounds on
the diameter of the hypergraph as a function of the second smallest eigenvalue
of the Laplacian operator. The Markov process underlying the Laplacian operator
can be viewed as a dispersion process on the vertices of the hypergraph that
might be of independent interest. We bound the {\em Mixing-time} of this
process as a function of the second smallest eigenvalue of the Laplacian
operator. All these results are generalizations of the corresponding results
for graphs.
We show that there can be no linear operator for hypergraphs whose spectra
captures hypergraph expansion in a Cheeger-like manner. For any , we give a
polynomial time algorithm to compute an approximation to the smallest
eigenvalue of the operator. We show that this approximation factor is optimal
under the SSE hypothesis (introduced by \cite{rs10}) for constant values of
.
Finally, using the factor preserving reduction from vertex expansion in
graphs to hypergraph expansion, we show that all our results for hypergraphs
extend to vertex expansion in graphs
Tectonic and stratigraphic evolution based on seismic sequence stratigraphy: Central rift section of the campos basin, offshore brazil
The rift section of the Brazilian basins represent the sedimentary record associated with the first stages of Gondwana breakâup in the Early Cretaceous phase (Berriasian to Aptian). The rift succession of the Campos Basin constitutes one of the main petroleum systems of Brazilâs marginal basins. This interval contains the main source rock and important reservoirs in the Lagoa Feia Group deposits. The Lagoa Feia Group is characterized by siliciclastic, carbonate and evaporite sediments deposited during the rift and postârift phases. Despite the economic relevance, little is known in stratigraphic terms regarding this rift interval. To date, most studies of the Lagoa Feia Group have adopted a lithostratigraphic approach, while this study proposes a tectonostrati-graphic framework for the deepârift succession of the Campos Basin (Lagoa Feia Group), using the fundamentals of seismic sequence stratigraphy. This work also aims to establish a methodological and practical procedure for the stratigraphic analysis of rift basins, using seismic data and seismofacies, and focusing on tectonicstratigraphic analysis. The dataset comprised 2D seismic lines, core and lithological logs from exploration wells. Three seismic facies were identified based on reflector patterns and lithologic data from well cores, providing an improved subdivision of the preâ, synâ and postârift stages. The synârift stage was further subdivided based on the geometric patterns of the reflectors. Tectonics was the main controlling factor in the sedimentary succession, and the pattern and geometry of the seismic reflectors of the synârift interval in the Campos Basin allowed the identification of three tectonic systems tracts: (i) a Rift Initiation Systems Tract; (ii) a High Tectonic Activity Systems Tract and (iii) a Low Tectonic Activity Systems Tract
Examination of staphylococcus aureus prophages circulating in Egypt
Staphylococcus aureus infections are of growing concern given the increased incidence of antibiotic resistant strains. Egypt, like several other countries, has seen alarming increases in methicillin-resistant S. aureus (MRSA) infections. This species can rapidly acquire genes associated with resistance, as well as virulence factors, through mobile genetic elements, including phages. Re-cently, we sequenced 56 S. aureus genomes from Alexandria Main University Hospital in Alexandria, Egypt, complementing 17 S. aureus genomes publicly available from other sites in Egypt. In the current study, we found that the majority (73.6%) of these strains contain intact prophages, including Biseptimaviruses, Phietaviruses, and Triaviruses. Further investigation of these prophages revealed evidence of horizontal exchange of the integrase for two of the prophages. These Egyptian S. aureus prophages are predicted to encode numerous virulence factors, including genes associated with immune evasion and toxins, including the PantonâValentine leukocidin (PVL)-associated genes lukF-PV/lukS-PV. Thus, prophages are likely to be a major contributor to the virulence of S. aureus strains in circulation in Egypt
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