14,213 research outputs found
dbar-approach to the dispersionless KP hierarchy
The dispersionless limit of the scalar nonlocal dbar-problem is derived. It
is given by a special class of nonlinear first-order equations. A
quasi-classical version of the dbar-dressing method is presented. It is shown
that the algebraic formulation of dispersionless hierarchies can be expressed
in terms of properties of Beltrami tupe equations. The universal Whitham
hierarchy and, in particular, the dispersionless KP hierarchy turn out to be
rings of symmetries for the quasi-classical dbar-problem.Comment: 13 pages, LaTex 24.9K
Wheel–rail contact: experimental study of the creep forces–creepage relationships
The wheel–rail contact problem plays an important role in the simulation methods used to solve railway dynamics problems. As a consequence, many different mathematical models have been developed to calculate wheel–rail contact forces. However, most of them tackle this problem purely from a theoretical point of view and need to be experimentally validated. Such validation could also reveal the influence of certain parameters not taken into account in the mathematical developments. This paper presents the steps followed in building a scaled test-bench to experimentally characterise the wheel–rail contact problem. The results of the longitudinal contact force as a function of the longitudinal creepage are obtained and the divergences with respect to Kalker's simplified theory are analysed. The influence of lateral creepage, angular velocity and certain contaminants such as cutting fluid or high positive friction modifier is also discussed
Geiger-Mode Avalanche Photodiodes in Particle Detection
It is well known that avalanche photodiodes operated in the Geiger mode above
the breakdown voltage offer a virtually infinite sensitivity and time accuracy
in the picosecond range that can be used for single photon detection. However,
their performance in particle detection remains still unexplored. In this
contribution, we are going to expose the different steps that we have taken in
order to prove the efficiency of Geiger mode avalanche photodiodes in the
aforementioned field. In particular, we will present an array of pixels of
1mmx1mm fabricated with a standard CMOS technology for characterization in a
test beam.Comment: 7 pages, 2 figures, Proceedings of LCWS1
Identifying Retweetable Tweets with a Personalized Global Classifier
In this paper we present a method to identify tweets that a user may find
interesting enough to retweet. The method is based on a global, but
personalized classifier, which is trained on data from several users,
represented in terms of user-specific features. Thus, the method is trained on
a sufficient volume of data, while also being able to make personalized
decisions, i.e., the same post received by two different users may lead to
different classification decisions. Experimenting with a collection of approx.\
130K tweets received by 122 journalists, we train a logistic regression
classifier, using a wide variety of features: the content of each tweet, its
novelty, its text similarity to tweets previously posted or retweeted by the
recipient or sender of the tweet, the network influence of the author and
sender, and their past interactions. Our system obtains F1 approx. 0.9 using
only 10 features and 5K training instances.Comment: This is a long paper version of the extended abstract titled "A
Personalized Global Filter To Predict Retweets", of the same authors, which
was published in the 25th ACM UMAP conference in Bratislava, Slovakia, in
July 201
Noise in neurons is message-dependent
Neuronal responses are conspicuously variable. We focus on one particular
aspect of that variability: the precision of action potential timing. We show
that for common models of noisy spike generation, elementary considerations
imply that such variability is a function of the input, and can be made
arbitrarily large or small by a suitable choice of inputs. Our considerations
are expected to extend to virtually any mechanism of spike generation, and we
illustrate them with data from the visual pathway. Thus, a simplification
usually made in the application of information theory to neural processing is
violated: noise {\sl is not independent of the message}. However, we also show
the existence of {\sl error-correcting} topologies, which can achieve better
timing reliability than their components.Comment: 6 pages,6 figures. Proceedings of the National Academy of Sciences
(in press
Globally Guided Trajectory Planning in Dynamic Environments
Navigating mobile robots through environments shared with humans is
challenging. From the perspective of the robot, humans are dynamic obstacles
that must be avoided. These obstacles make the collision-free space nonconvex,
which leads to two distinct passing behaviors per obstacle (passing left or
right). For local planners, such as receding-horizon trajectory optimization,
each behavior presents a local optimum in which the planner can get stuck. This
may result in slow or unsafe motion even when a better plan exists. In this
work, we identify trajectories for multiple locally optimal driving behaviors,
by considering their topology. This identification is made consistent over
successive iterations by propagating the topology information. The most
suitable high-level trajectory guides a local optimization-based planner,
resulting in fast and safe motion plans. We validate the proposed planner on a
mobile robot in simulation and real-world experiments.Comment: 7 pages, 6 figures, accepted to IEEE International Conference on
Robotics and Automation (ICRA) 202
Testable axion-like particles in the minimal linear σ model
Axion and axion-like particle models are typically affected by a strong fine-tuning problem in conceiving the electroweak and the Peccei-Quinn breaking scales. Within the context of the Minimal Linear σ Model, axion-like particle constructions are identified where this hierarchy problem is solved, accounting for a TeV Peccei-Quinn breaking scale and a pseudoscalar particle with a mass larger than 10 MeV. Potential signatures at colliders and B-factories are discussedThe authors acknowledge partial financial support by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreements No.690575 and No. 674896. J.A.G and L.M. acknowledge partial financial support by the Spanish Agencia Estatal de Investigación (AEI) and the EU Fondo Europeo de Desarrollo Regional(FEDER) through the project FPA2016-78645-P, and through the Centro de excelencia Severo Ochoa Program under grant SEV-2016-0597. L. M. acknowledges partial financial support by the Spanish MINECO through the “Ramón y Cajal” programme (RYC-2015-17173).J.A.G. and L.M. thank the Physics and Astronomy department “Galileo Galilei” of the Padua University for hospitality during the development of this project. Furthermore, L.M. thanks the Kavli Institute for the Physics and Mathematics of the Universe for hospitality during the development of this projec
Identificação das oportunidades de inovação em APL por multicritério grupo de Pesquisa: economia e gestão no agronegócio.
Solutions of the dispersion equation in the region of overlapping of zero-sound and particle-hole modes
In this paper the solutions of the zero-sound dispersion equation in the
random phase approximation (RPA) are considered. The calculation of the damped
zero-sound modes \omega_s(k) (complex frequency of excitation) in the nuclear
matter is presented. The method is based on the analytical structure of the
polarization operators \Pi(\omega,k). The solutions of two dispersion equations
with \Pi(\omega,k) and with Re(\Pi(\omega,k)) are compared. It is shown that in
the first case we obtain one-valued smooth solutions without "thumb-like"
forms. Considering the giant resonances in the nuclei as zero-sound excitations
we compare the experimental energy and escape width of the giant dipole
resonance (GDR) in the nucleus A with \omega_s(k) taken at a definite wave
vector k=k_A.Comment: 14 pages, 5 figures; revised versio
Dynamics of zonal flow-like structures in the edge of the TJ-II stellarator
The dynamics of fluctuating electric field structures in the edge of the
TJ-II stellarator, that display zonal flow-like traits, is studied. These
structures have been shown to be global and affect particle transport
dynamically [J.A. Alonso et al., Nucl. Fus. 52 063010 (2012)]. In this article
we discuss possible drive (Reynolds stress) and damping (Neoclassical
viscosity, geodesic transfer) mechanisms for the associated ExB velocity. We
show that: (a) while the observed turbulence-driven forces can provide the
necessary perpendicular acceleration, a causal relation could not be firmly
established, possibly because of the locality of the Reynolds stress
measurements, (b) the calculated neoclassical viscosity and damping times are
comparable to the observed zonal flow relaxation times, and (c) although an
accompanying density modulation is observed to be associated to the zonal flow,
it is not consistent with the excitation of pressure side-bands, like those
present in geodesic acoustic oscillations, caused by the compression of the ExB
flow field
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