9,921 research outputs found
Algebraic Invariance Conditions in the Study of Approximate (Null-)Controllability of Markov Switch Processes
We aim at studying approximate null-controllability properties of a
particular class of piecewise linear Markov processes (Markovian switch
systems). The criteria are given in terms of algebraic invariance and are
easily computable. We propose several necessary conditions and a sufficient
one. The hierarchy between these conditions is studied via suitable
counterexamples. Equivalence criteria are given in abstract form for general
dynamics and algebraic form for systems with constant coefficients or
continuous switching. The problem is motivated by the study of lysis phenomena
in biological organisms and price prediction on spike-driven commodities.Comment: Mathematics of Control, Signals, and Systems, Springer Verlag
(Germany), 2015, online first
http://link.springer.com/article/10.1007/s00498-015-0146-
A scheme for simulating one-dimensional diffusion processes with discontinuous coefficients
The aim of this article is to provide a scheme for simulating diffusion
processes evolving in one-dimensional discontinuous media. This scheme does not
rely on smoothing the coefficients that appear in the infinitesimal generator
of the diffusion processes, but uses instead an exact description of the
behavior of their trajectories when they reach the points of discontinuity.
This description is supplied with the local comparison of the trajectories of
the diffusion processes with those of a skew Brownian motion.Comment: Published at http://dx.doi.org/10.1214/105051605000000656 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
A Piecewise Deterministic Markov Toy Model for Traffic/Maintenance and Associated Hamilton-Jacobi Integrodifferential Systems on Networks
We study optimal control problems in infinite horizon when the dynamics
belong to a specific class of piecewise deterministic Markov processes
constrained to star-shaped networks (inspired by traffic models). We adapt the
results in [H. M. Soner. Optimal control with state-space constraint. II. SIAM
J. Control Optim., 24(6):1110.1122, 1986] to prove the regularity of the value
function and the dynamic programming principle. Extending the networks and
Krylov's ''shaking the coefficients'' method, we prove that the value function
can be seen as the solution to a linearized optimization problem set on a
convenient set of probability measures. The approach relies entirely on
viscosity arguments. As a by-product, the dual formulation guarantees that the
value function is the pointwise supremum over regular subsolutions of the
associated Hamilton-Jacobi integrodifferential system. This ensures that the
value function satisfies Perron's preconization for the (unique) candidate to
viscosity solution. Finally, we prove that the same kind of linearization can
be obtained by combining linearization for classical (unconstrained) problems
and cost penalization. The latter method works for very general near-viable
systems (possibly without further controllability) and discontinuous costs.Comment: accepted to Applied Mathematics and Optimization (01/10/2015
INSA scientific activities in the space astronomy area
Support to Astronomy operations is an important and long-lived activity
within INSA. Probably the best known (and traditional) INSA activities are
those related with real-time spacecraft operations: Ground station maintenance
and operation (Ground station engineers and operators); spacecraft and payload
real-time operation (spacecraft and instruments controllers); computing
infrastructure maintenance (operators, analysts) and general site services.In
this paper, we'll show a different perspective, probably not so well-known,
presenting some INSA recent activities at the European Space Astronomy Centre
(ESAC) and NASA Madrid Deep Space Communication Complex (MDSCC) directly
related to scientific operations. Basic lines of activity involved include:
Operations support for science operations; system and software support for real
time systems; technical administration and IT support; R \& D activities,
radioastronomy (at MDSCC and ESAC) and scientific research projects. This paper
is structured as follows: first, INSA activities in two ESA cornerstone
astrophysics missions, XMM-Newton and Herschel, will be outlined. Then, our
activities related to Science infrastructure services, represented by the
Virtual Observatory (VO) framework and the Science Archives development
facilities are briefly shown. Radio Astronomy activities will be described
afterwards, and finally, a few research topics in which INSA scientists are
involved will be also described.Comment: 6 pages. Highlights of Spanish Astrophysics V Proceedings of the VIII
Scientific Meeting of the Spanish AstronomicalSociety (SEA) held in
Santander, 7-11 July, 200
Elaboración y caracterización de concreto polimérico: efecto del contenido de fibra de luffa y de la radiación gamma
A pesar de que el concreto convencional (elaborado con cemento Portland) es un material con gran versatilidad en cuanto a su funcionalidad y economía, presenta algunas limitaciones tales como: alta porosidad y baja resistencia a esfuerzos de flexión, tensión y a reactivos químicos altamente corrosivos. Algunas de estas limitaciones se han subsanado mediante la incorporación de fibras. Se han podido mejorar la resistencia a la tracción, compresión, durabilidad, resistencia a la fatiga, resistencia al impacto y a la abrasión, la contracción, la expansión, la estabilidad térmica, y resistencia al fuego. Sin embargo, muy pocas investigaciones ha abordado el efecto de fibras en concreto poliméricoEn el presente trabajo se estudian los efectos de la incorporación de fibras y de la radiación gamma en las propiedades mecánicas del concreto polimérico, en particular las resistencias a la compresión y a la flexión, la deformación mecánica y los módulos de elasticidad. Los concretos son elaborados con resina de poliéster insaturada, arena sílice y fibras de Luffa como agregados.CONACY
Payments for Environmental Services in Watersheds: Insights From a Comparative Study of three Cases in Central America
We have compared three cases of payments for water-related environmental services (PES) in Central America, in terms of socioeconomic background, opportunity costs of forest conservation and stakeholders’ perceptions on the conditions of water resources and other issues. We found that, in general, the foregone benefits from land uses alternative to forest cover are larger than the amount paid, which apparently contradicts the economic foundation of PES schemes. A number of possible explanations are explored. The results also suggest that trade-offs between different environmental and social goals are likely to emerge in PES schemes, posing some doubts on their ability to be multipurpose instruments for environmental improvement and rural development. We also found that PES schemes may work as a conflictresolution instrument, facilitating downstream -upstream problem solving, though at the same time they might introduce changes in social perceptions of property rights.environmental services, watershed management, rural development, property rights, Honduras, Costa Rica, Nicaragua.
Liquid-crystal patterns of rectangular particles in a square nanocavity
Using density-functional theory in the restricted-orientation approximation,
we analyse the liquid-crystal patterns and phase behaviour of a fluid of hard
rectangular particles confined in a two-dimensional square nanocavity of side
length composed of hard inner walls. Patterning in the cavity is governed
by surface-induced order, capillary and frustration effects, and depends on the
relative values of particle aspect ratio , with the
length and the width of the rectangles (), and cavity size
. Ordering may be very different from bulk () behaviour when
is a few times the particle length (nanocavity). Bulk and confinement
properties are obtained for the cases , 3 and 6. In the confined
fluid surface-induced frustration leads to four-fold symmetry breaking in all
phases (which become two-fold symmetric). Since no director distorsion can
arise in our model by construction, frustration in the director orientation is
relaxed by the creation of domain walls (where the director changes by
); this configuration is necessary to stabilise periodic phases.
For the crystal becomes stable with commensuration transitions
taking place as is varied. In the case the commensuration
transitions involve columnar phases with different number of columns. Finally,
in the case , the high-density region of the phase diagram is
dominated by commensuration transitions between smectic structures; at lower
densities there is a symmetry-breaking isotropic nematic transition
exhibiting non-monotonic behaviour with cavity size.Comment: 31 pages, 15 figure
Simulating Spiking Neural P systems without delays using GPUs
We present in this paper our work regarding simulating a type of P system
known as a spiking neural P system (SNP system) using graphics processing units
(GPUs). GPUs, because of their architectural optimization for parallel
computations, are well-suited for highly parallelizable problems. Due to the
advent of general purpose GPU computing in recent years, GPUs are not limited
to graphics and video processing alone, but include computationally intensive
scientific and mathematical applications as well. Moreover P systems, including
SNP systems, are inherently and maximally parallel computing models whose
inspirations are taken from the functioning and dynamics of a living cell. In
particular, SNP systems try to give a modest but formal representation of a
special type of cell known as the neuron and their interactions with one
another. The nature of SNP systems allowed their representation as matrices,
which is a crucial step in simulating them on highly parallel devices such as
GPUs. The highly parallel nature of SNP systems necessitate the use of hardware
intended for parallel computations. The simulation algorithms, design
considerations, and implementation are presented. Finally, simulation results,
observations, and analyses using an SNP system that generates all numbers in
- {1} are discussed, as well as recommendations for future work.Comment: 19 pages in total, 4 figures, listings/algorithms, submitted at the
9th Brainstorming Week in Membrane Computing, University of Seville, Spai
Analysis of information use in agricultural science PhD theses at Central University of Venezuela
Purpose – To analyze information use in agricultural science PhD theses submitted between 1986 and 2002 in the Faculty of Agronomy, Central University of Venezuela (UCV).
Design/methodology/approach – The source of information was the UCV Faculty of Agronomy, Library's database, “Tesis”. The unidimensional production and use indicators analyzed included: scientific production, reference density, self-citations, document contemporaneousness and type of documents cited, reference scattering and accessibility of the journals cited.
Findings – The analysis of the data obtained from 4,646 bibliographic references in 42 agricultural science PhD theses provides insight into information use in a Venezuelan agricultural science community. The mean number of references per thesis found was 113?±?21. The number of women earning a PhD in agriculture was observed to grow. The percentage of self-citations varied widely. The half-life was 11 years and the Price's Index 22 per cent. According to the distribution by document type, most of the publications cited were articles in journals, while references to technical standards and internet publications were rare. UCV Faculty of Agronomy PhD students tended to seek information primarily in the Anglo Saxon literature. The Celestino Bonfanti Library periodicals section met a high proportion (92 per cent) of the demand for journals located in the first and second concentration-scattering zones.
Originality/value – This is the only paper on the evaluation of PhD theses in Venezuela. The findings will be useful for education planners in Venezuela and other developing countries.Publicad
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