1,188 research outputs found
Path-Fault-Tolerant Approximate Shortest-Path Trees
Let be an -nodes non-negatively real-weighted undirected graph.
In this paper we show how to enrich a {\em single-source shortest-path tree}
(SPT) of with a \emph{sparse} set of \emph{auxiliary} edges selected from
, in order to create a structure which tolerates effectively a \emph{path
failure} in the SPT. This consists of a simultaneous fault of a set of at
most adjacent edges along a shortest path emanating from the source, and it
is recognized as one of the most frequent disruption in an SPT. We show that,
for any integer parameter , it is possible to provide a very sparse
(i.e., of size ) auxiliary structure that carefully
approximates (i.e., within a stretch factor of ) the true
shortest paths from the source during the lifetime of the failure. Moreover, we
show that our construction can be further refined to get a stretch factor of
and a size of for the special case , and that it can be
converted into a very efficient \emph{approximate-distance sensitivity oracle},
that allows to quickly (even in optimal time, if ) reconstruct the
shortest paths (w.r.t. our structure) from the source after a path failure,
thus permitting to perform promptly the needed rerouting operations. Our
structure compares favorably with previous known solutions, as we discuss in
the paper, and moreover it is also very effective in practice, as we assess
through a large set of experiments.Comment: 21 pages, 3 figures, SIROCCO 201
Fully-dynamic Approximation of Betweenness Centrality
Betweenness is a well-known centrality measure that ranks the nodes of a
network according to their participation in shortest paths. Since an exact
computation is prohibitive in large networks, several approximation algorithms
have been proposed. Besides that, recent years have seen the publication of
dynamic algorithms for efficient recomputation of betweenness in evolving
networks. In previous work we proposed the first semi-dynamic algorithms that
recompute an approximation of betweenness in connected graphs after batches of
edge insertions.
In this paper we propose the first fully-dynamic approximation algorithms
(for weighted and unweighted undirected graphs that need not to be connected)
with a provable guarantee on the maximum approximation error. The transfer to
fully-dynamic and disconnected graphs implies additional algorithmic problems
that could be of independent interest. In particular, we propose a new upper
bound on the vertex diameter for weighted undirected graphs. For both weighted
and unweighted graphs, we also propose the first fully-dynamic algorithms that
keep track of such upper bound. In addition, we extend our former algorithm for
semi-dynamic BFS to batches of both edge insertions and deletions.
Using approximation, our algorithms are the first to make in-memory
computation of betweenness in fully-dynamic networks with millions of edges
feasible. Our experiments show that they can achieve substantial speedups
compared to recomputation, up to several orders of magnitude
Open and / or laparoscopic surgical treatment of liver hydatic cysts
Hydatid disease is a severe parasitic disease with a widely ranging distribution. In the human being the liver is the most frequent organ affected. 1 The treatment should be individualized to the morphology, size, number and location of the cysts, that is why a variety of surgical operations have been advocated from complete resection like total pericystectomy or partial hepatectomy to laparoscopy to a minimally invasive procedures like percutaneous aspiration of cysts to conservative drug therapy. 3-4 This study compares laparoscopic versus open management of the hydatid cyst of liver the surgical approach to liver echinococcosis is still a controversial issue and shows our results of surgical treatment of liver hydatid cysts during a 3-years period
Personal identity (de)formation among lifestyle travellers: A double-edged sword?
This article explores the personal identity work of lifestyle travellers – individuals for whom extended leisure travel is a preferred lifestyle that they return to repeatedly. Qualitative findings from in-depth semi-structured interviews with lifestyle travellers in northern India and southern Thailand are interpreted in light of theories on identity formation in late modernity that position identity as problematic. It is suggested that extended leisure travel can provide exposure to varied cultural praxes that may contribute to a sense of social saturation. Whilst a minority of the respondents embraced a saturation of personal identity in the subjective formation of a cosmopolitan cultural identity, several of the respondents were paradoxically left with more identity questions than answers as the result of their travels
The challenge of enterprise/innovation: a case study of a modern university
In the prevailing economic and political climate for Higher Education a greater emphasis has been placed on diversifying the funding base. The present study was undertaken between 2012 and 2014 and addressed the implementation of an approach to the transformation of one academic school in a medium-sized modern university in Wales to a more engaged enterprise culture. A multimethod investigation included a bi-lingual (English and Welsh) online survey of academic staff and yielded a 71% response rate (n = 45). The findings informed a series of in-depth interviews (n = 24) with a representative sample of those involved in enterprise work (support staff, managers, senior managers), and those who were not. The results provided the platform for the ‘S4E model’ for effective engagement with enterprise: (1) Strategic significance for Enterprise, (2) Support for Enterprise, (3) Synergy for Enterprise, and (4) Success for Enterprise. The outcomes of the research and the recommendations from it have potential to inform practice in other academic schools within the university and, in a wider context, within other Schools of Education regionally, nationally and internationally. Its original empirical exploration of enterprise within education studies is a significant contribution to that body of knowledge
Prospects in Constraining the Dark Energy Potential
We generalize to non-flat geometries the formalism of Simon et al. (2005) to
reconstruct the dark energy potential. This formalism makes use of quantities
similar to the Horizon-flow parameters in inflation, can, in principle, be made
non-parametric and is general enough to be applied outside the simple, single
scalar field quintessence. Since presently available and forthcoming data do
not allow a non-parametric and exact reconstruction of the potential, we
consider a general parametric description in term of Chebyshev polynomials. We
then consider present and future measurements of H(z), Baryon Acoustic
Oscillations surveys and Supernovae type 1A surveys, and investigate their
constraints on the dark energy potential. We find that, relaxing the flatness
assumption increases the errors on the reconstructed dark energy evolution but
does not open up significant degeneracies, provided that a modest prior on
geometry is imposed. Direct measurements of H(z), such as those provided by BAO
surveys, are crucially important to constrain the evolution of the dark energy
potential and the dark energy equation of state, especially for non-trivial
deviations from the standard LambdaCDM model.Comment: 22 pages, 7 figures. 2 references correcte
Forward Global Photometric Calibration of the Dark Energy Survey
Many scientific goals for the Dark Energy Survey (DES) require calibration of
optical/NIR broadband photometry that is stable in time and uniform
over the celestial sky to one percent or better. It is also necessary to limit
to similar accuracy systematic uncertainty in the calibrated broadband
magnitudes due to uncertainty in the spectrum of the source. Here we present a
"Forward Global Calibration Method (FGCM)" for photometric calibration of the
DES, and we present results of its application to the first three years of the
survey (Y3A1). The FGCM combines data taken with auxiliary instrumentation at
the observatory with data from the broad-band survey imaging itself and models
of the instrument and atmosphere to estimate the spatial- and time-dependence
of the passbands of individual DES survey exposures. "Standard" passbands are
chosen that are typical of the passbands encountered during the survey. The
passband of any individual observation is combined with an estimate of the
source spectral shape to yield a magnitude in the standard
system. This "chromatic correction" to the standard system is necessary to
achieve sub-percent calibrations. The FGCM achieves reproducible and stable
photometric calibration of standard magnitudes of stellar
sources over the multi-year Y3A1 data sample with residual random calibration
errors of per exposure. The accuracy of the
calibration is uniform across the DES footprint to
within . The systematic uncertainties of magnitudes in
the standard system due to the spectra of sources are less than
for main sequence stars with .Comment: 25 pages, submitted to A
Search for Sub-TeV Gamma Rays Coincident with BATSE Gamma Ray Bursts
Project GRAND is a 100m x 100m air shower array of proportional wire chambers
(PWCs). There are 64 stations each with eight 1.29 m^2 PWC planes arranged in
four orthogonal pairs placed vertically above one another to geometrically
measure the angles of charged secondaries. A steel plate above the bottom pair
of PWCs differentiates muons (which pass undeflected through the steel) from
non-penetrating particles. FLUKA Monte Carlo studies show that a TeV gamma ray
striking the atmosphere at normal incidence produces 0.23 muons which reach
ground level where their angles and identities are measured. Thus,
paradoxically, secondary muons are used as a signature for gamma ray primaries.
The data are examined for possible angular and time coincidences with eight
gamma ray bursts (GRBs) detected by BATSE. Seven of the GRBs were selected
because of their good acceptance by GRAND and high BATSE Fluence. The eighth
GRB was added due to its possible coincident detection by Milagrito. For each
of the eight candidate GRBs, the number of excess counts during the BATSE T90
time interval and within plus or minus five degrees of BATSE's direction was
obtained. The highest statistical significance reported in this paper (2.7
sigma) is for the event that was predicted to be the most likely to be observed
(GRB 971110).Comment: To be presented at the XXVIII International Cosmic Ray Conference,
Tsukuba, Japa
NTT and NOT spectroscopy of SDSS-II supernovae
Context. The SDSS-II Supernova Survey, conducted between 2005 and 2007, was
designed to detect a large number of Type Ia supernovae (SNe Ia) around z~0.2,
the redshift "gap" between low-z and high-z SN searches. The survey has
provided multi-band photometric lightcurves for variable targets, and SN
candidates were scheduled for spectroscopic observations, primarily to provide
SN classification and accurate redshifts. We present SN spectra obtained in
2006 and 2007 using the NTT and the NOT. Aims. We provide an atlas of SN
spectra in the range z =0.03-0.32 that complements the well-sampled lightcurves
from SDSS-II in the forthcoming three-year SDSS SN cosmology analysis. The
sample can, for example, be used for spectral studies of SNe Ia, which are
critical for understanding potential systematic effects when SNe are used to
determine cosmological distances. Methods. The spectra were reduced in a
uniform manner, and special care was taken in estimating the uncertainties for
the different processing steps. Host-galaxy light was subtracted when possible
and the SN type fitted using the SuperNova IDentification code (SNID). We also
present comparisons between spectral and photometric dating using SALT
lightcurve fits to the photometry from SDSS-II, as well as the global
distribution of our sample in terms of the lightcurve parameters: stretch and
colour. Results. We report new spectroscopic data from 141 SNe Ia, mainly
between -9 and +15 days from lightcurve maximum, including a few cases of
multi-epoch observations. This homogeneous, host-galaxy subtracted, SN Ia
spectroscopic sample is among the largest such data sets and unique in its
redshift interval. The sample includes two potential SN 1991T-like SNe (SN
2006on and SN 2007ni) and one potential SN 2002cx-like SN (SN 2007ie). In
addition, the new compilation includes spectra from 23 confirmed Type II and 8
Type Ib/c SNe.Comment: Accepted for publication in A&
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