19,784 research outputs found
Comparing the hierarchy of author given tags and repository given tags in a large document archive
Folksonomies - large databases arising from collaborative tagging of items by
independent users - are becoming an increasingly important way of categorizing
information. In these systems users can tag items with free words, resulting in
a tripartite item-tag-user network. Although there are no prescribed relations
between tags, the way users think about the different categories presumably has
some built in hierarchy, in which more special concepts are descendants of some
more general categories. Several applications would benefit from the knowledge
of this hierarchy. Here we apply a recent method to check the differences and
similarities of hierarchies resulting from tags given by independent
individuals and from tags given by a centrally managed repository system. The
results from out method showed substantial differences between the lower part
of the hierarchies, and in contrast, a relatively high similarity at the top of
the hierarchies.Comment: 10 page
Wormholes, naked singularities and universes of ghost radiation
Both the static and homogeneous metrics describing the spherically symmetric
gravitational field of a crossflow of incoming and outgoing null dust streams
are generalized for the case of the two-component ghost radiation. Static
solutions represent either naked singularities or the wormholes recently found
by Hayward. The critical value of the parameter separating the two
possibilities is given. The wormhole is allowed to have positive mass. The
homogeneous solutions are open universes.Comment: 5 pages, 10 figures, minor changes to match the published versio
Vacuum Kerr-Schild metrics generated by nontwisting congruences
The Kerr-Schild pencil of metrics , with
and satisfying the vacuum Einstein equations, is
investigated in the case when the null vector has vanishing twist. This
class of Kerr-Schild metrics contains two solutions: the Kasner metric and a
metric wich can be obtained from the Kasner metric by a complex coordinate
transformation. Both are limiting cases of the K\'ota-Perj\'es metrics. The
base space-time is a pp-wave.Comment: 12 page
Gravitational dynamics in a 2+1+1 decomposed spacetime along nonorthogonal double foliations: Hamiltonian evolution and gauge fixing
Motivated by situations with temporal evolution and spatial symmetries both
singled out, we develop a new 2+1+1 decomposition of spacetime, based on a
nonorthogonal double foliation. Time evolution proceeds along the leaves of the
spatial foliation. We identify the gravitational variables in the velocity
phase-space as the 2-metric (induced on the intersection of
the hypersurfaces of the foliations), the 2+1 components of the spatial shift
vector, together with the extrinsic curvature, normal fundamental form and
normal fundamental scalar of , all constructed with the
normal to the temporal foliation. This work generalizes a previous
decomposition based on orthogonal foliations, a formalism lacking one metric
variable, now reintroduced. The new metric variable is related to (i) the angle
of a Lorentz-rotation between the nonorthogonal bases adapted to the
foliations, and (ii) to the vorticity of these basis vectors. As a first
application of the formalism, we work out the Hamiltonian dynamics of general
relativity in terms of the variables identified as canonical, generalizing
previous work. As a second application we present the unambiguous gauge-fixing
suitable to discuss the even sector scalar-type perturbations of spherically
symmetric and static spacetimes in generic scalar-tensor gravitational
theories, which has been obstructed in the formalism of orthogonal double
foliation.Comment: 16 pages, 4 figures, to appear in Phys. Rev.
3+1+1 dimensional covariant gravitational dynamics on an asymmetrically embedded brane
We give the evolution and constraint equations on an asymmetrically embedded
brane in the form of average and difference equations.Comment: 7 pages; due to page limits first two sections to be published in the
Proceedings of the Grassmannian Conference in Fundamental Cosmology
(Grasscosmofun'09), Szczecin, Poland 2009, Ann. Physik; third section in the
Proceedings of the Twelfth Marcel Grossmann Meeting, Paris, France 200
Modal Empiricism and Two-Dimensional Semantics
In the paper I argue that it follows from David Chalmers’ semantics and metaphysics concerning microphysical terms that microphysical identifications are strongly necessary. This result supports modal empiricism and also counts in favour of a posteriori materialism (and thus against Chalmers’ property dualism), for it blocks the conjecture that a posteriori materialism is committed to strong necessities, but there are no such modalities
Extracting tag hierarchies
Tagging items with descriptive annotations or keywords is a very natural way
to compress and highlight information about the properties of the given entity.
Over the years several methods have been proposed for extracting a hierarchy
between the tags for systems with a "flat", egalitarian organization of the
tags, which is very common when the tags correspond to free words given by
numerous independent people. Here we present a complete framework for automated
tag hierarchy extraction based on tag occurrence statistics. Along with
proposing new algorithms, we are also introducing different quality measures
enabling the detailed comparison of competing approaches from different
aspects. Furthermore, we set up a synthetic, computer generated benchmark
providing a versatile tool for testing, with a couple of tunable parameters
capable of generating a wide range of test beds. Beside the computer generated
input we also use real data in our studies, including a biological example with
a pre-defined hierarchy between the tags. The encouraging similarity between
the pre-defined and reconstructed hierarchy, as well as the seemingly
meaningful hierarchies obtained for other real systems indicate that tag
hierarchy extraction is a very promising direction for further research with a
great potential for practical applications.Comment: 25 pages with 21 pages of supporting information, 25 figure
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