11,816 research outputs found
On Byzantine Broadcast in Loosely Connected Networks
We consider the problem of reliably broadcasting information in a multihop
asynchronous network that is subject to Byzantine failures. Most existing
approaches give conditions for perfect reliable broadcast (all correct nodes
deliver the authentic message and nothing else), but they require a highly
connected network. An approach giving only probabilistic guarantees (correct
nodes deliver the authentic message with high probability) was recently
proposed for loosely connected networks, such as grids and tori. Yet, the
proposed solution requires a specific initialization (that includes global
knowledge) of each node, which may be difficult or impossible to guarantee in
self-organizing networks - for instance, a wireless sensor network, especially
if they are prone to Byzantine failures. In this paper, we propose a new
protocol offering guarantees for loosely connected networks that does not
require such global knowledge dependent initialization. In more details, we
give a methodology to determine whether a set of nodes will always deliver the
authentic message, in any execution. Then, we give conditions for perfect
reliable broadcast in a torus network. Finally, we provide experimental
evaluation for our solution, and determine the number of randomly distributed
Byzantine failures than can be tolerated, for a given correct broadcast
probability.Comment: 1
A Scalable Byzantine Grid
Modern networks assemble an ever growing number of nodes. However, it remains
difficult to increase the number of channels per node, thus the maximal degree
of the network may be bounded. This is typically the case in grid topology
networks, where each node has at most four neighbors. In this paper, we address
the following issue: if each node is likely to fail in an unpredictable manner,
how can we preserve some global reliability guarantees when the number of nodes
keeps increasing unboundedly ? To be more specific, we consider the problem or
reliably broadcasting information on an asynchronous grid in the presence of
Byzantine failures -- that is, some nodes may have an arbitrary and potentially
malicious behavior. Our requirement is that a constant fraction of correct
nodes remain able to achieve reliable communication. Existing solutions can
only tolerate a fixed number of Byzantine failures if they adopt a worst-case
placement scheme. Besides, if we assume a constant Byzantine ratio (each node
has the same probability to be Byzantine), the probability to have a fatal
placement approaches 1 when the number of nodes increases, and reliability
guarantees collapse. In this paper, we propose the first broadcast protocol
that overcomes these difficulties. First, the number of Byzantine failures that
can be tolerated (if they adopt the worst-case placement) now increases with
the number of nodes. Second, we are able to tolerate a constant Byzantine
ratio, however large the grid may be. In other words, the grid becomes
scalable. This result has important security applications in ultra-large
networks, where each node has a given probability to misbehave.Comment: 17 page
AGN Feedback Compared: Jets versus Radiation
Feedback by Active Galactic Nuclei is often divided into quasar and radio
mode, powered by radiation or radio jets, respectively. Both are fundamental in
galaxy evolution, especially in late-type galaxies, as shown by cosmological
simulations and observations of jet-ISM interactions in these systems. We
compare AGN feedback by radiation and by collimated jets through a suite of
simulations, in which a central AGN interacts with a clumpy, fractal galactic
disc. We test AGN of and erg/s, considering jets
perpendicular or parallel to the disc. Mechanical jets drive the more powerful
outflows, exhibiting stronger mass and momentum coupling with the dense gas,
while radiation heats and rarifies the gas more. Radiation and perpendicular
jets evolve to be quite similar in outflow properties and effect on the cold
ISM, while inclined jets interact more efficiently with all the disc gas,
removing the densest in Myr, and thereby reducing the amount of
cold gas available for star formation. All simulations show small-scale inflows
of M/yr, which can easily reach down to the Bondi radius of
the central supermassive black hole (especially for radiation and perpendicular
jets), implying that AGN modulate their own duty cycle in a feedback/feeding
cycle.Comment: 21 pages, 15 figures, 2 table
An unconditionally stable staggered algorithm for transient finite element analysis of coupled thermoelastic problems
An unconditionally stable second order accurate implicit-implicit staggered procedure for the finite element solution of fully coupled thermoelasticity transient problems is proposed. The procedure is stabilized with a semi-algebraic augmentation technique. A comparative cost analysis reveals the superiority of the proposed computational strategy to other conventional staggered procedures. Numerical examples of one and two-dimensional thermomechanical coupled problems demonstrate the accuracy of the proposed numerical solution algorithm
Mutual productivity spillovers between foreign and local firms in China
The existing literature treats advanced technology sourcing as the only cause of reverse productivity spillovers from local to foreign firms and implies that mutual spillovers between foreign and local firms can only happen in the developed world. This paper argues that the diffusion of indigenous technology and local knowledge helps the productivity enhancement of multinationals, so that there can be mutual spillovers even in a developing country. The results from a large-sample firm-level econometric analysis and a comparative case study of seven companies in Chinese manufacturing support this new argument, as mutual spillovers are identified between local Chinese firms and overseas Chinese or OECD-invested firms
Field induced anisotropic cooperativity in a magnetic colloidal glass
The translational dynamics in a repulsive colloidal glass-former is probed by
time-resolved X-ray Photon Correlation Spectroscopy. In this dense dispersion
of charge-stabilized and magnetic nanoparticles, the interaction potential can
be tuned, from quasi-isotropic to anisotropic by applying an external magnetic
field. Structural and dynamical anisotropies are reported on interparticle
lengthscales associated with highly anisotropic cooperativity, almost two
orders of magnitude larger in the field direction than in the perpendicular
direction and in zero field
Ultra-low phase noise all-optical microwave generation setup based on commercial devices
In this paper, we present a very simple design based on commercial devices
for the all-optical generation of ultra-low phase noise microwave signals. A
commercial, fibered femtosecond laser is locked to a laser that is stabilized
to a commercial ULE Fabry-Perot cavity. The 10 GHz microwave signal extracted
from the femtosecond laser output exhibits a single sideband phase noise
at 1 Hz Fourier frequency, at
the level of the best value obtained with such "microwave photonics" laboratory
experiments \cite{Fortier2011}. Close-to-the-carrier ultra-low phase noise
microwave signals will now be available in laboratories outside the frequency
metrology field, opening up new possibilities in various domains.Comment: 8 pages, 3 figures. To be published in Applied Optics, early posting
version available at
http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=23114
Massive spheroids can form in single minor mergers
Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of âex situâ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M â > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11â0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (âŒ5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers â or any significant merging at all â contrary to what is classically believed.Peer reviewedFinal Accepted Versio
Observation of superspin glass state in magnetically textured ferrofluid (gamma-Fe2O3)
Magnetic properties in a magnetically textured ferrofluid made out of
interacting maghemite (gamma-Fe2O3) nanoparticles suspended in glycerin have
been investigated. Despite the loss of uniform distribution of anisotropy axes,
a superspin glass state exists at low temperature in a concentrated, textured
ferrofluid as in the case of its non-textured counterpart. The onset of
superspin glass state was verified from the sample's AC susceptibility. The
influence of the anisotropy axis orientation on the aging behavior in the
glassy states is also discussed
COMMENTS ABOUT HIGGS FIELDS, NONCOMMUTATIVE GEOMETRY AND THE STANDARD MODEL
We make a short review of the formalism that describes Higgs and Yang Mills
fields as two particular cases of an appropriate generalization of the notion
of connection. We also comment about the several variants of this formalism,
their interest, the relations with noncommutative geometry, the existence (or
lack of existence) of phenomenological predictions, the relation with Lie
super-algebras etc.Comment: pp 20, LaTeX file, no figures, also available via anonymous ftp at
ftp://cpt.univ-mrs.fr/ or via gopher gopher://cpt.univ-mrs.fr
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