683 research outputs found
Distributed Random Process for a Large-Scale Peer-to-Peer Lottery
Most online lotteries today fail to ensure the verifiability of the random
process and rely on a trusted third party. This issue has received little
attention since the emergence of distributed protocols like Bitcoin that
demonstrated the potential of protocols with no trusted third party. We argue
that the security requirements of online lotteries are similar to those of
online voting, and propose a novel distributed online lottery protocol that
applies techniques developed for voting applications to an existing lottery
protocol. As a result, the protocol is scalable, provides efficient
verification of the random process and does not rely on a trusted third party
nor on assumptions of bounded computational resources. An early prototype
confirms the feasibility of our approach
Improving the mass determination of Galactic Cepheids
We have selected a sample of Galactic Cepheids for which accurate estimates
of radii, distances, and photometric parameters are available. The comparison
between their pulsation masses, based on new Period-Mass-Radius (PMR)
relations, and their evolutionary masses, based on both optical and NIR
Color-Magnitude (CM) diagrams, suggests that pulsation masses are on average of
the order of 10% smaller than the evolutionary masses. Current pulsation masses
show, at fixed radius, a strongly reduced dispersion when compared with values
published in literature.The increased precision in the pulsation masses is due
to the fact that our predicted PMR relations based on nonlinear, convective
Cepheid models present smaller standard deviations than PMR relations based on
linear models. At the same time, the empirical radii of our Cepheid sample are
typically accurate at the 5% level. Our evolutionary mass determinations are
based on stellar models constructed by neglecting the effect of mass-loss
during the He burning phase. Therefore, the difference between pulsation and
evolutionary masses could be intrinsic and does not necessarily imply a problem
with either evolutionary and/or nonlinear pulsation models. The marginal
evidence of a trend in the difference between evolutionary and pulsation masses
when moving from short to long-period Cepheids is also briefly discussed. The
main finding of our investigation is that the long-standing Cepheid mass
discrepancy seems now resolved at the 10% level either if account for canonical
or mild convective core overshooting evolutionary models.Comment: 14 pages, 4 postscript figures, accepted for publication on ApJ
Letter
The escape problem under stochastic volatility: the Heston model
We solve the escape problem for the Heston random diffusion model. We obtain
exact expressions for the survival probability (which ammounts to solving the
complete escape problem) as well as for the mean exit time. We also average the
volatility in order to work out the problem for the return alone regardless
volatility. We look over these results in terms of the dimensionless normal
level of volatility --a ratio of the three parameters that appear in the Heston
model-- and analyze their form in several assymptotic limits. Thus, for
instance, we show that the mean exit time grows quadratically with large spans
while for small spans the growth is systematically slower depending on the
value of the normal level. We compare our results with those of the Wiener
process and show that the assumption of stochastic volatility, in an apparent
paradoxical way, increases survival and prolongs the escape time.Comment: 29 pages, 12 figure
Reverse wedge osteotomy of the distal radius in Madelung's deformity
Madelung\u27s deformity results from a growth defect in the palmar and ulnar region of the distal radius. It presents as an excessively inclined radial joint surface, inducing "spontaneous progressive palmar subluxation of the wrist". The principle of reverse wedge osteotomy (RWO) consists in the reorientation of the radial joint surface by taking a circumferential bone wedge, the base of which is harvested from the excess of the radial and dorsal cortical bone of the distal radius, then turning it over and putting back this reverse wedge into the osteotomy so as to obtain closure on the excess and opening on the deficient cortical bone. RWO corrects the palmar subluxation of the carpus and improves distal radio-ulnar alignment. All five bilaterally operated patients were satisfied, esthetically and functionally. Its corrective power gives RWO a place apart among the surgical techniques currently available in Madelung\u27s deformity
Peer-to-Peer Secure Multi-Party Numerical Computation Facing Malicious Adversaries
We propose an efficient framework for enabling secure multi-party numerical
computations in a Peer-to-Peer network. This problem arises in a range of
applications such as collaborative filtering, distributed computation of trust
and reputation, monitoring and other tasks, where the computing nodes is
expected to preserve the privacy of their inputs while performing a joint
computation of a certain function. Although there is a rich literature in the
field of distributed systems security concerning secure multi-party
computation, in practice it is hard to deploy those methods in very large scale
Peer-to-Peer networks. In this work, we try to bridge the gap between
theoretical algorithms in the security domain, and a practical Peer-to-Peer
deployment.
We consider two security models. The first is the semi-honest model where
peers correctly follow the protocol, but try to reveal private information. We
provide three possible schemes for secure multi-party numerical computation for
this model and identify a single light-weight scheme which outperforms the
others. Using extensive simulation results over real Internet topologies, we
demonstrate that our scheme is scalable to very large networks, with up to
millions of nodes. The second model we consider is the malicious peers model,
where peers can behave arbitrarily, deliberately trying to affect the results
of the computation as well as compromising the privacy of other peers. For this
model we provide a fourth scheme to defend the execution of the computation
against the malicious peers. The proposed scheme has a higher complexity
relative to the semi-honest model. Overall, we provide the Peer-to-Peer network
designer a set of tools to choose from, based on the desired level of security.Comment: Submitted to Peer-to-Peer Networking and Applications Journal (PPNA)
200
The Photometric and Kinematic Structure of Face-On Disk Galaxies. I. Sample Definition, H-alpha Integral Field Spectroscopy, and HI Line-Widths
We present a survey of the photometric and kinematic properties of 39 nearby,
nearly face-on disk galaxies. Our approach exploits echelle-resolution
integral-field spectroscopy of the H-alpha regions, obtained with DensePak on
the WIYN 3.5m telescope Bench Spectrograph. This data is complemented by HI
line-profiles observed with the Nancay radio telescope for 25 of these sample
galaxies. Twelve additional line-widths are available for sample galaxies from
the literature. In this paper, we introduce the goals of this survey, define
the sample selection algorithm, and amass the integral field spectroscopic data
and HI line-widths. We establish spatially-integrated H-alpha line-widths for
the sample. We test the veracity of these spatially-integrated line profiles by
convolving narrow-band imaging data with velocity field information for one of
the sample galaxies, PGC 38268, and also by comparing to HI line profiles. We
find HI and H-alpha line profiles to be similar in width but different in
shape, indicating we are observing different spatial distributions of ionized
and neutral gas in largely axisymmetric systems with flat outer
rotation-curves. We also find vertical velocity dispersions of the ionized disk
gas within several disk scale-lengths have a median value of 18 km/s and an 80%
range of 12-26 km/s. This is only a factor of ~2 larger than what is observed
for neutral atomic and molecular gas. With standard assumptions for intrinsic
and thermal broadening for H-alpha, this translates into a factor of three
range in turbulent velocities, between 8 and 25 km/s.Comment: 29 pages, 20 figures; accepted for publication in ApJ Supplement
Serie
A Wide Field Survey of Satellite Galaxies around the Spiral Galaxy M106
We present a wide field survey of satellite galaxies in M106 (NGC 4258)
covering a 1.7\degr \times 2\degr field around M106 using
Canada-France-Hawaii Telescope/MegaCam. We find 16 satellite galaxy candidates
of M106.
Eight of these galaxies are found to be dwarf galaxies that are much smaller
and fainter than the remaining galaxies. Four of these galaxies are new
findings. Surface brightness profiles of 15 out of 16 satellite galaxies can be
represented well by an exponential disk profile with varying scale length. We
derive the surface number density distribution of these satellite galaxies. The
central number density profile (d kpc) is well fitted by a power-law
with a power index of , similar to the expected power index of
isothermal distribution. The luminosity function of these satellites is
represented well by the Schechter function with a faint end slope of
. Integrated photometric properties (total luminosity,
total colour, and disk scale length) and the spatial distribution of these
satellite galaxies are found to be roughly similar to those of the Milky Way
and M31.Comment: Accepted by MNRA
Near-Infrared Photometric Survey of Proto-Planetary Nebula Candidates
We present JHK' photometric measurements of 78 objects mostly consisting of
proto-planetary nebula candidates. Photometric magnitudes are determined by
means of imaging and aperture photometry. Unlike the observations with a
photometer with a fixed-sized beam, the method of imaging photometry permits
accurate derivation of photometric values because the target sources can be
correctly identified and confusion with neighboring sources can be easily
avoided. Of the 78 sources observed, we report 10 cases in which the source
seems to have been misidentified or confused by nearby bright sources. We also
present nearly two dozen cases in which the source seems to have indicated a
variability which prompts a follow-up monitoring. There are also a few sources
that show previously unreported extendedness. In addition, we present H band
finding charts of the target sources.Comment: 3 tables, 1 figur
The Kr85 s-process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type red
giants. These objects belong to the class of Asymptotic Giant Branch stars,
experiencing thermal instabilities in the He-burning shell (thermal pulses).
Mixing episodes called third dredge-up enrich the photosphere with newly
synthesized C12 in the He-rich zone, and this is the source of the high
observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic
abundance estimates confirm that, in agreement with the general understanding
of the late evolutionary stages of low and intermediate mass stars, carbon
enrichment is accompanied by the appearance of s-process elements in the
photosphere. We discuss the details of the observations and of the derived
abundances, focusing in particular on rubidium, a neutron-density sensitive
element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The
critical reaction branching at Kr85, which determines the relative enrichment
of the studied species, is discussed. Subsequently, we compare our data with
recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch
stars, at metallicities relevant for our sample. A remarkable agreement between
model predictions and observations is found. Thanks to the different neutron
density prevailing in low and intermediate mass stars, comparison with the
models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We
also analyze the C12/C13 ratios measured, showing that most of them cannot be
explained by canonical stellar models. We discuss how this fact would require
the operation of an ad hoc additional mixing, currently called Cool Bottom
Process, operating only in low mass stars during the first ascent of the red
giant branch and, perhaps, also during the asymptotic giant branch.Comment: 54 pages + 6 figures + 6 tables. ApJ accepte
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