41,782 research outputs found
Taxonomy and clustering in collaborative systems: the case of the on-line encyclopedia Wikipedia
In this paper we investigate the nature and structure of the relation between
imposed classifications and real clustering in a particular case of a
scale-free network given by the on-line encyclopedia Wikipedia. We find a
statistical similarity in the distributions of community sizes both by using
the top-down approach of the categories division present in the archive and in
the bottom-up procedure of community detection given by an algorithm based on
the spectral properties of the graph. Regardless the statistically similar
behaviour the two methods provide a rather different division of the articles,
thereby signaling that the nature and presence of power laws is a general
feature for these systems and cannot be used as a benchmark to evaluate the
suitability of a clustering method.Comment: 5 pages, 3 figures, epl2 styl
A hybrid computer program for rapidly solving flowing or static chemical kinetic problems involving many chemical species
A hybrid chemical kinetic computer program was assembled which provides a rapid solution to problems involving flowing or static, chemically reacting, gas mixtures. The computer program uses existing subroutines for problem setup, initialization, and preliminary calculations and incorporates a stiff ordinary differential equation solution technique. A number of check cases were recomputed with the hybrid program and the results were almost identical to those previously obtained. The computational time saving was demonstrated with a propane-oxygen-argon shock tube combustion problem involving 31 chemical species and 64 reactions. Information is presented to enable potential users to prepare an input data deck for the calculation of a problem
Low radio frequency signatures of ram pressure stripping in Virgo spiral NGC 4254
We report the detection of extended low radio frequency continuum emission
beyond the optical disk of the spiral galaxy NGC 4254 using the Giant Metrewave
Radio Telescope. NGC 4254, which has an almost face-on orientation, is located
in the outskirts of the Virgo cluster. Since such extended emission is uncommon
in low inclination galaxies, we believe it is a signature of magnetised plasma
pushed out of the disk by ram pressure of the intracluster medium as NGC 4254
falls into the Virgo cluster. The detailed spectral index distribution across
NGC 4254 shows that the steepest spectrum alpha < -1 (S propto nu^{alpha})
arises in the gas beyond the optical disk. This lends support to the ram
pressure scenario by indicating that the extended emission is not from the disk
gas but from matter which has been stripped by ram pressure. The steeper
spectrum of the extended emission is reminiscent of haloes in edge-on galaxies.
The sharp fall in intensity and enhanced polarization in the south of the
galaxy, in addition to enhanced star formation reported by others provide
evidence towards the efficacy of ram pressure on this galaxy. HI 21cm
observations show that the gas in the north lags in rotation and hence is
likely the atomic gas which is carried along with the wind. NGC 4254 is a
particularly strong radio emitter with a power of 7x10^{22} Watts/Hz at 240
MHz. We find that the integrated spectrum of the galaxy flattens at lower
frequencies and is well explained by an injection spectrum with
alpha_0=-0.45+-0.12. We end by comparing published simulation results with our
data and conclude that ram pressure stripping is likely to be a significant
contributor to evolution of galaxies residing in X-ray poor groups and cluster
outskirts.Comment: 10 pages, 7 figures (figures 1,2 and 4 are in jpg format) and 2
tables; Accepted for publication in MNRA
Geometry of good sets in n-fold Cartesian product
We propose here a multidimensional generalisation of the notion of link
introduced in our previous papers and we discuss some consequences for
simplicial measures and sums of function algebras.Comment: 17 pages, no figures, no table
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