3,046 research outputs found
TK: The Twitter Top-K Keywords Benchmark
Information retrieval from textual data focuses on the construction of
vocabularies that contain weighted term tuples. Such vocabularies can then be
exploited by various text analysis algorithms to extract new knowledge, e.g.,
top-k keywords, top-k documents, etc. Top-k keywords are casually used for
various purposes, are often computed on-the-fly, and thus must be efficiently
computed. To compare competing weighting schemes and database implementations,
benchmarking is customary. To the best of our knowledge, no benchmark currently
addresses these problems. Hence, in this paper, we present a top-k keywords
benchmark, TK, which features a real tweet dataset and queries with
various complexities and selectivities. TK helps evaluate weighting
schemes and database implementations in terms of computing performance. To
illustrate TK's relevance and genericity, we successfully performed
tests on the TF-IDF and Okapi BM25 weighting schemes, on one hand, and on
different relational (Oracle, PostgreSQL) and document-oriented (MongoDB)
database implementations, on the other hand
Popular critiques of consultancy and a politics of management learning?
In this short article, I argue that popular business discourse on the role of management consultancy in the promotion and translation of management ideas is often critical, informed by more or less implicit ethical and political concerns with employee security, equity, openness and the transparency and legitimacy of responsibility. These concerns are, in part, âsayableâ because their object is seen as a scapegoat for management. Nevertheless, combined with the popular form of their expression, they can support and legitimize critical studies of management learning, a discipline which otherwise has become overly concerned with processual and situational phenomena at the expense of broader political dynamics and of the content and consequences of management and management knowledg
Direct measurement of molecular stiffness and damping in confined water layers
We present {\em direct} and {\em linear} measurements of the normal stiffness
and damping of a confined, few molecule thick water layer. The measurements
were obtained by use of a small amplitude (0.36 ), off-resonance
Atomic Force Microscopy (AFM) technique. We measured stiffness and damping
oscillations revealing up to 7 layers separated by 2.56 0.20
. Relaxation times could also be calculated and were found to
indicate a significant slow-down of the dynamics of the system as the confining
separation was reduced. We found that the dynamics of the system is determined
not only by the interfacial pressure, but more significantly by solvation
effects which depend on the exact separation of tip and surface. Thus `
solidification\rq seems to not be merely a result of pressure and confinement,
but depends strongly on how commensurate the confining cavity is with the
molecule size. We were able to model the results by starting from the simple
assumption that the relaxation time depends linearly on the film stiffness.Comment: 7 pages, 6 figures, will be submitted to PR
Population III star formation in a Lambda CDM universe, I: The effect of formation redshift and environment on protostellar accretion rate
(abridged) We perform 12 extremely high resolution adaptive mesh refinement
cosmological hydrodynamic simulations of Population III star formation in a
Lambda CDM universe, varying the box size and large-scale structure, to
understand systematic effects in the formation of primordial protostellar
cores. We find results that are qualitatively similar to those observed
previously. We observe that the threshold halo mass for formation of a
Population III protostar does not evolve significantly with time in the
redshift range studied (33 > z > 19) but exhibits substantial scatter due to
different halo assembly histories: Halos which assembled more slowly develop
cooling cores at lower mass than those that assemble more rapidly, in agreement
with Yoshida et al. (2003). We do, however, observe significant evolution in
the accretion rates of Population III protostars with redshift, with objects
that form later having higher maximum accretion rates, with a variation of two
orders of magnitude (10^-4 - 10^-2 Msolar/year). This can be explained by
considering the evolving virial properties of the halos with redshift and the
physics of molecular hydrogen formation at low densities. Our result implies
that the mass distribution of Population III stars inferred from their
accretion rates may be broader than previously thought, and may evolve with
redshift. Finally, we observe that our collapsing protostellar cloud cores do
not fragment, consistent with previous results, which suggests that Population
III stars which form in halos of mass 10^5 - 10^6 Msun always form in
isolation.Comment: Accepted by The Astrophysical Journal. Some minor changes. 65 pages,
3 tables, 21 figures (3 color). To appear in January 1, 2007 issu
Phase Mixing of Alfvén Waves Near a 2D Magnetic Null Point
The propagation of linear Alfvén wave pulses in an inhomogeneous plasma near a 2D coronal null point is investigated. When a uniform plasma density is considered, it is seen that an initially planar Alfvén wavefront remains planar, despite the varying equilibrium Alfvén speed, and that all the wave collects at the separatrices. Thus, in the non-ideal case, these Alfvénic disturbances preferentially dissipate their energy at these locations. For a non-uniform equilibrium density, it is found that the Alfvén wavefront is significantly distorted away from the initially planar geometry, inviting the possibility of dissipation due to phase mixing. Despite this however, we conclude that for the Alfvén wave, current density accumulation and preferential heating still primarily occur at the separatrices, even when an extremely non-uniform density profile is considered
Evolution of high-frequency gravitational waves in some cosmological models
We investigate Isaacson's high-frequency gravitational waves which propagate
in some relevant cosmological models, in particular the FRW spacetimes. Their
time evolution in Fourier space is explicitly obtained for various metric forms
of (anti--)de Sitter universe. Behaviour of high-frequency waves in the
anisotropic Kasner spacetime is also described.Comment: 14 pages, 8 figures, to appear in Czech. J. Phy
What is the true nature of blinkers?
Aims.
The aim of this work is to identify the true nature of the transient EUV brightenings, called blinkers.
Methods.
Co-spatial and co-temporal multi-instrument data, including imaging (EUVI/STEREO, XRT and SOT/Hinode), spectroscopic (CDS/SoHO and EIS/Hinode) and magnetogram (SOT/Hinode) data, of an isolated equatorial coronal hole were used. An automatic program for identifying transient brightenings in CDS O v 629 Ă
, EUVI 171 Ă
and XRT was applied.
Results.
We identified 28 blinker groups in the CDS O v 629 Ă
raster images. All CDS O v 629 Ă
blinkers showed counterparts in EUVI 171 Ă
and 304 Ă
images. We classified these blinkers into two categories, one associated with coronal counterparts and other with no coronal counterparts as seen in XRT images and EIS Fe xii 195.12 Ă
raster images. Around two-thirds of the blinkers show coronal counterparts and correspond to various events like EUV/X-ray jets, brightenings in coronal bright points or foot-point brightenings of larger loops. These brightenings occur repetitively and have a lifetime of around 40 min at transition region temperatures. The remaining blinker groups with no coronal counterpart in XRT and EIS Fe xii 195.12 Ă
appear as point-like brightenings and have chromospheric/transition region origin. They take place only once and have a lifetime of around 20 min. In general, lifetimes of blinkers are different at different wavelengths, i.e. different temperatures, decreasing from the chromosphere to the corona.
Conclusions.
This work shows that the term blinker covers a range of phenomena. Blinkers are the EUV response of various transient events originating at coronal, transition region and chromospheric heights. Hence, events associated with blinkers contribute to the formation and maintenance of the temperature gradient in the transition region and the corona
Comparing AMR and SPH Cosmological Simulations: I. Dark Matter & Adiabatic Simulations
We compare two cosmological hydrodynamic simulation codes in the context of
hierarchical galaxy formation: The SPH code GADGET, and the Eulerian AMR code
ENZO. Both codes represent dark matter with the N-body method, but use
different gravity solvers and fundamentally different approaches to
hydrodynamics. We compare the GADGET `entropy conserving' SPH formulation with
two ENZO methods: The piecewise parabolic method (PPM), and the artificial
viscosity-based scheme used in the ZEUS code. In this paper we focus on a
comparison of cosmological simulations that follow either only dark matter, or
also adiabatic baryonic gas. The dark matter-only runs agree generally quite
well, provided ENZO is run with a comparatively fine root grid and a low
overdensity threshold for mesh refinement, otherwise the abundance of low-mass
halos is suppressed. This is due to the hierarchical particle-mesh method used
to compute gravitational forces in ENZO, which tends to deliver lower force
resolution than the tree algorithm of GADGET. At comparable force resolution,
we find that the latter offers substantially better performance and lower
memory consumption than the present gravity solver in ENZO. In simulations that
include adiabatic gas dynamics, we find general agreement in the distribution
functions of temperature, entropy, and density for gas of moderate to high
overdensity, as found inside dark matter halos. However, there are some
significant differences at lower overdensities. We argue that these
discrepancies are presumably owing to differences in the shock-capturing
abilities of the different methods. In particular, ZEUS hydro leads to some
unphysical heating at early times in preshock regions. Overall, the GADGET
hydro results are bracketed by those for ENZO/ZEUS and ENZO/PPM. (abridged)Comment: 45 pages, 17 figures. ApJ, in press. This paper has been
significantly revised after referee comments and several sections have been
adde
Genetic resistance determinants to fusidic acid and chlorhexidine in variably susceptible staphylococci from dogs
Abstract
Background
Concern exists that frequent use of topically-applied fusidic acid (FA) and chlorhexidine (CHX) for canine pyoderma is driving clinically relevant resistance, despite rare description of FA and CHX genetic resistance determinants in canine-derived staphylococci. This study aimed to determine minimum inhibitory concentrations (MICs) and investigate presence of putative resistance determinants for FA and CHX in canine-derived methicillin-resistant (MR) and -susceptible (MS) staphylococci. Plasmid-mediated resistance genes (fusB, fusC, fusD, qacA/B, smr; PCR) and MICs (agar dilution) of FA and CHX were investigated in 578 staphylococci (50 MR S. aureus [SA], 50 MSSA, 259 MR S. pseudintermedius [SP], 219 MSSP) from Finland, U.S.A., North (NUK) and South-East U.K. (SEUK) and Germany. In all isolates with FA MIC â„64âmg/L (n =â27) fusA and fusE were amplified and sequenced.
Results
FA resistance determinants (fusA mutations n =â24, fusB nâ=â2, fusC n =â36) were found in isolates from all countries bar U.S.A. and correlated with higher MICs (â„1âmg/L), although 4 SP isolates had MICs of 0.06âmg/L despite carrying fusC. CHX MICs did not correlate with qacA/B (nâ=â2) and smr (n =â5), which were found in SEUK SA, and SP from NUK and U.S.A.
Conclusions
Increased FA MICs were frequently associated with fusA mutations and fusC, and this is the first account of fusB in SP. Despite novel description of qacA/B in SP, gene presence did not correlate with CHX MIC. Selection pressure from clinical use might increase prevalence of these genetic determinants, but clinical significance remains uncertain in relation to high skin concentrations achieved by topical therapy
Fabrication and characterisation of nanocrystalline graphite MEMS resonators using a geometric design to control buckling
The simulation, fabrication and characterisation of nanographite MEMS resonators is reported in this paper. The deposition of nanographite is achieved using plasma-enhanced chemical vapour deposition directly onto numerous substrates such as commercial silicon wafers. As a result, many of the reliability issues of devices based on transferred graphene are avoided. The fabrication of the resonators is presented along with a simple undercutting method to overcome buckling, by changing the effective stress of the structure from 436âMPa compressive, to 13âMPa tensile. The characterisation of the resonators using electrostatic actuation and laser Doppler vibrometry is reported, demonstrating resonator frequencies from 5â640âkHz and quality factor above 1819 in vacuum obtained
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