3,951 research outputs found
Finding lower bounds on the complexity of secret sharing schemes by linear programming
Optimizing the maximum, or average, length of the shares in relation to the length of the secret for every given access structure is a difficult and long-standing open problem in cryptology. Most of the known lower bounds on these parameters have been obtained by implicitly or explicitly using that every secret sharing scheme defines a polymatroid related to the access structure. The best bounds that can be obtained by this combinatorial method can be determined by using linear programming, and this can be effectively done for access structures on a small number of participants.
By applying this linear programming approach, we improve some of the known lower bounds for the access structures on five participants and the graph access structures on six participants for which these parameters were still undetermined. Nevertheless, the lower bounds that are obtained by this combinatorial method are not tight in general. For some access structures, they can be improved by adding to the linear program non-Shannon information inequalities as new constraints. We obtain in this way new separation results for some graph access structures on eight participants and for some ports of non-representable matroids. Finally, we prove that, for two access structures on five participants, the combinatorial lower bound cannot be attained by any linear secret sharing schemePeer ReviewedPostprint (author's final draft
An attractor for dark matter structures
Cosmological simulations of dark matter structures have identified a set of
universal profiles, and similar characteristics have been seen in
non-cosmological simulations. It has therefore been speculated whether these
profiles of collisionless systems relate to accretion and merger history, or if
there is an attractor for the dark matter systems. Here we identify such a
1-dimensional attractor in the 3-dimensional space spanned by the 2 radial
slopes of the density and velocity dispersion, and the velocity anisotropy.
This attractor effectively removes one degree of freedom from the Jeans
equation. It also allows us to speculate on a new fluid interpretation for the
Jeans equation, with an effective polytropic index for the dark matter
particles between 1/2 and 3/4. If this attractor solution holds for other
collisionless structures, then it may hold the key to break the mass-anisotropy
degeneracy, which presently prevents us from measuring the mass profiles in
dwarf galaxies uniquely.Comment: 7 pages, 2 figures, comments welcom
An observational case study of synergies between an intense heat wave and the urban heat island in Beijing
The focus of this study is an intense heat episode that occurred on 9–13 July 2017 in Beijing, China, that resulted in severe impacts on natural and human variables, including record-setting daily electricity consumption levels. This event was observed and analyzed with a suite of local and mesoscale instruments, including a high-density automated weather station network, soil moisture sensors, and ground-based vertical instruments (e.g., a wind profiler, a ceilometer, and three radiometers) situated in and around the city, as well as electric power consumption data and analysis data from the U.S. National Centers for Environmental Prediction. The results show that the heat wave originated from dry adiabatic warming induced by the dy-namic downslope and synoptic subsidence. The conditions were aggravated by the increased air humidity during subsequent days, which resulted in historically high records of the heat index (i.e., an index repre-senting the apparent temperature that incorporates both air temperature and moisture). The increased thermal energy and decreased boundary layer height resulted in a highly energized urban boundary layer. The differences between urban and rural thermal conditions throughout almost the entire boundary layer were enhanced during the heat wave, and the canopy-layer urban heat island intensity (UHII) reached up to 8°C at a central urban station at 2300 local standard time 10 July. A double-peak pattern in the diurnal cycle of UHIIs occurred during the heat wave and differed from the single-peak pattern of the decadal average UHII cycles. Different spatial distributions of UHII values occurred during the day and night
Posthuman geographies in Patagonia-Ays\ue9n: thresholds, shores and intersections between animal ∩ human<strong> </strong>
CSIndicators: Get tailored climate indicators for applications in your sector
CSIndicators is an R package that gathers generalised methods for the flexible computation of climate-related indicators. Each method represents a specific mathematical approach which is combined with the possibility of selecting a flexible time period to define the indicator. This enables a wide range of possibilities for tailoring indicators to sectorial climate service applications. This package is intended for sub-seasonal, seasonal and decadal climate predictions, but its methods are also applicable to other time scales. Additionally, this package is compatible with the CSTools R package for climate forecast post-processing.This package was developed in the context of H2020 MED-GOLD (776467), S2S4E (776787), VITIGEOSS (869565) projects and Horizon Europe ASPECT project (101081460).Peer ReviewedPostprint (published version
Deep, Wide-field CCD Photometry for the Open Cluster NGC3532
We present the results of a deep, wide-field CCD survey for the open cluster
NGC~3532. Our new photometry effectively covers a one square
degree area and reaches an unprecedented depth of to reveal that
NGC~3532 is a rich open cluster that harbors a large number of faint, low-mass
stars. We employ a number of methods to reduce the impact of field star
contamination in the cluster color-magnitude diagrams, including supplementing
our photometry with data from the 2MASS catalog. These efforts allow
us to define a robust sample of candidate main sequence stars suitable for a
purely empirical determination of the cluster's parameters by comparing them to
the well-established Hyades main sequence. Our results confirm previous
findings that NGC~3532 lies fairly near to the Sun [;
~pc] and has an extremely low reddening for its location near
the Galactic plane []. Moreover, an age of \,Myr
has been derived for the cluster by fitting a set of overshooting isochrones to
the well-populated upper main-sequence. This new photometry also extends faint
enough to reach the cluster white dwarf sequence, as confirmed by our
photometric recovery of eight spectroscopically identified members of the
cluster. Using the location of these eight members, along with the latest
theoretical cooling tracks, we have identified additional white dwarf
stars in the color-magnitude diagram that have a high probability
of belonging to NGC~3532. The age we derive from fitting white dwarf isochrones
to the locus of these stars, \,Myr, is consistent with the age
derived from the turnoff. Our analysis of the photometry also includes an
estimation of the binary star fraction, as well as a determination of the
cluster's luminosity and mass functions.Comment: Accepted for publication in AJ. 55 pages, 21 figures. High-quality
version with complete data tables can be downloaded from
http://www.phys.lsu.edu/~jclem/NGC3532
The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
We study how chemical abundances of late B-, A- and early F-type stars evolve
with time, and we search for correlations between the abundance of chemical
elements and other stellar parameters, such as effective temperature and Vsini.
We have observed a large number of B-, A- and F-type stars belonging to open
clusters of different ages. In this paper we concentrate on the Praesepe
cluster (log t = 8.85), for which we have obtained high resolution, high
signal-to-noise ratio spectra of sixteen normal A- and F-type stars and one Am
star, using the SOPHIE spectrograph of the Observatoire de Haute-Provence. For
all the observed stars, we have derived fundamental parameters and chemical
abundances. In addition, we discuss another eight Am stars belonging to the
same cluster, for which the abundance analysis had been presented in a previous
paper. We find a strong correlation between peculiarity of Am stars and Vsini.
The abundance of the elements underabundant in Am stars increases with Vsini,
while it decreases for the overabundant elements. Chemical abundances of
various elements appear correlated with the iron abundance.Comment: Accepted for publication on A&
Indication of Superconductivity at 35 K in Graphite-Sulfur Composites
We report magnetization measurements performed on graphite--sulfur composites
which demonstrate a clear superconducting behavior below the critical
temperature T = 35 K. The Meissner-Ochsenfeld effect, screening
supercurrents, and magnetization hysteresis loops characteristic of type-II
superconductors were measured. The results indicate that the superconductivity
occurs in a small sample fraction, possibly related to the sample surface.Comment: published versio
MIUSCAT: extended MILES spectral coverage. I. Stellar populations synthesis models
We extend the spectral range of our stellar population synthesis models based
on the MILES and CaT empirical stellar spectral libraries. For this purpose we
combine these two libraries with the Indo-U.S. to construct composite stellar
spectra to feed our models. The spectral energy distributions (SEDs) computed
with these models and the originally published models are combined to construct
composite SEDs for single-age, single-metallicity stellar populations (SSPs)
covering the range 3465 - 9469\AA at moderately high, and uniform, resolution
(FWHM=2.51\AA). The colours derived from these SSP SEDs provide good fits to
Galactic globular cluster data. We find that the colours involving redder
filters are very sensitive to the IMF, as well as a number of features and
molecular bands throughout the spectra. To illustrate the potential use of
these models we focus on the NaI doublet at 8200 \AA and with the aid of the
newly synthesized SSP model SEDs we define a new IMF-sensitive index that is
based on this feature, which overcomes various limitations from previous index
definitions for low velocity dispersion stellar systems. We propose an
index-index diagram based on this feature and the neighboring CaII triplet at
8600 \AA, to constrain the IMF if the age and [Na/Fe] abundance are known.
Finally we also show a survey-oriented spectrophotometric application which
evidences the accurate flux calibration of these models for carrying out
reliable spectral fitting techniques. These models are available through our
user-friendly website.Comment: 16 pages, 17 figures, 2 tables; MNRAS in press. Model predictions
available at our website: http://miles.iac.e
Environmental variables, habitat discontinuity and life history shaping the genetic structure of Pomatoschistus marmoratus
Coastal lagoons are semi-isolated ecosystems
exposed to wide fluctuations of environmental conditions
and showing habitat fragmentation. These features may
play an important role in separating species into different
populations, even at small spatial scales. In this study, we
evaluate the concordance between mitochondrial (previous
published data) and nuclear data analyzing the genetic
variability of Pomatoschistus marmoratus in five localities,
inside and outside the Mar Menor coastal lagoon (SE
Spain) using eight microsatellites. High genetic diversity
and similar levels of allele richness were observed across
all loci and localities, although significant genic and
genotypic differentiation was found between populations
inside and outside the lagoon. In contrast to the FST values
obtained from previous mitochondrial DNA analyses
(control region), the microsatellite data exhibited significant
differentiation among samples inside the Mar Menor
and between lagoonal and marine samples. This pattern
was corroborated using Cavalli-Sforza genetic distances.
The habitat fragmentation inside the coastal lagoon and
among lagoon and marine localities could be acting as a
barrier to gene flow and contributing to the observed
genetic structure. Our results from generalized additive
models point a significant link between extreme lagoonal
environmental conditions (mainly maximum salinity) and
P. marmoratus genetic composition. Thereby, these environmental
features could be also acting on genetic structure
of coastal lagoon populations of P. marmoratus favoring
their genetic divergence. The mating strategy of P. marmoratus
could be also influencing our results obtained from
mitochondrial and nuclear DNA. Therefore, a special
consideration must be done in the selection of the DNA
markers depending on the reproductive strategy of the
species
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