3,967 research outputs found
Link prediction in very large directed graphs: Exploiting hierarchical properties in parallel
Link prediction is a link mining task that tries to find new edges within a given graph. Among the targets of link prediction there is large directed graphs, which are frequent structures nowadays. The typical sparsity of large graphs demands of high precision predictions in order to obtain usable results. However, the size of those graphs only permits the execution of scalable algorithms. As a trade-off between those two problems we recently proposed a link prediction algorithm for directed graphs that exploits hierarchical properties. The algorithm can be classified as a local score, which entails scalability. Unlike the rest of local scores, our proposal assumes the existence of an underlying model for the data which allows it to produce predictions with a higher precision. We test the validity of its hierarchical assumptions on two clearly hierarchical data sets, one of them based on RDF. Then we test it on a non-hierarchical data set based on Wikipedia to demonstrate its broad applicability. Given the computational complexity of link prediction in very large graphs we also introduce some general recommendations useful to make of link prediction an efficiently parallelized problem.Peer ReviewedPostprint (published version
Constraints on the phase diagram of molybdenum from first-principles free-energy calculations
We use first-principles techniques to re-examine the suggestion that
transitions seen in high-P experiments on Mo are solid-solid transitions from
the bcc structure to either the fcc or hcp structures. We confirm that in the
harmonic approximation the free energies of fcc and hcp structures become lower
than that of bcc at P > 325 GPa and T below the melting curve, as reported
recently. However, we show that if anharmonic effects are fully included this
is no longer true. We calculate fully anharmonic free energies of high-T
crystal phases by integration of the thermal average stress with respect to
strain as structures are deformed into each other, and also by thermodynamic
integration from harmonic reference systems to the fully anharmonic system. Our
finding that fcc is thermodynamically less stable than bcc in the relevant
high-P/high-T region is supported by comparing the melting curves of the two
structures calculated using the first-principles reference-coexistence
technique. We present first-principles simulations based on the recently
proposed Z method which also support the stability of bcc over fcc.Comment: 33 pages, 10 figure
Biology of growth of Hoplias aff. malabaricus (Bloch, 1794) in a shallow pampean lake Argentina
The trahira Hoplias aff. malabaricus is a top predator in pampean shallow lakes and is highly appreciated by recreational anglers and artisanal fishermen. Trahira growth from Yalca shallow lake was determined by lepidological analysis and age validated by marginal increment. When growth was fitted to the von Bertalanffy model, annual classes exhibited a bimodal pattern as a result of the presence of spring and summer annual cohorts associated with a three month spawning season, each period in turn showing different growth patterns. The trahira population-age structure at Yalca shallow lake showed a truncated profile with very low numbers of large adults and few individuals older than three to four years, thus producing an unbalanced length-structure population. Growth parameters and growth performance were similar to the corresponding parameters estimated for other shallow pampean lakes of the region, but strongly diverged from the data for those populations inhabiting subtropical and tropical environments. Such differences could be accounted for by dissimilarity in metabolic rates associated with thermal differences accompanying seasonal variability among latitudes as well as by the development of adaptive physiologic and demographic responses to cope with the high thermal amplitude and hydrologic instability observed in pampean lakes.A traíra Hoplias aff. malabaricus é um predador de topo encontrado em lagoas rasas nos Pampas, muito apreciada nas pescas comerciais e esportivas. O crescimento da traíra no lago Yalca foi determinado através de análises lepidológicas, validadas a partir do incremento marginal. O crescimento foi ajustado ao modelo de von Bertalanffy, sendo que as classes de tamanho anuais apresentaram padrão bimodal devido à presença de coortes de primavera e verão, associadas à uma longa estação reprodutiva com diferentes padrões de crescimento. A estrutura etária da traíra do lago Yalca apresentou um perfil truncado, com um número pequeno de adultos de maior porte e poucos indivíduos com mais de três ou quatro anos, produzindo uma estrutura em tamanho desbalanceada. Os parâmetros e a performance de crescimento foram similares aos estimados para outros lagos rasos da região, mas fortemente divergentes de populações encontradas em ambientes tropicais e subtropicais. Tais diferenças podem estar relacionadas à diferentes taxas metabólicas associadas à variação térmica sazonal entre latitudes e ao desenvolvimento de respostas adaptativas fisiológicas e demográficas em resposta aos padrões termais altamente variáveis e instabilidade hidrológica observados nos lagos da região dos Pampas.Fil: Balboni, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); ArgentinaFil: Colautti, Dario César. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); ArgentinaFil: Baigún, Claudio Rafael M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentin
Computational homogenization of fibrous piezoelectric materials
Flexible piezoelectric devices made of polymeric materials are widely used
for micro- and nano-electro-mechanical systems. In particular, numerous recent
applications concern energy harvesting. Due to the importance of computational
modeling to understand the influence that microscale geometry and constitutive
variables exert on the macroscopic behavior, a numerical approach is developed
here for multiscale and multiphysics modeling of thin piezoelectric sheets made
of aligned arrays of polymeric nanofibers, manufactured by electrospinning. At
the microscale, the representative volume element consists in piezoelectric
polymeric nanofibers, assumed to feature a piezoelastic behavior and subjected
to electromechanical contact constraints. The latter are incorporated into the
virtual work equations by formulating suitable electric, mechanical and
coupling potentials and the constraints are enforced by using the penalty
method. From the solution of the micro-scale boundary value problem, a suitable
scale transition procedure leads to identifying the performance of a
macroscopic thin piezoelectric shell element.Comment: 22 pages, 13 figure
A multiscale-multiphysics strategy for numerical modeling of thin piezoelectric sheets
Flexible piezoelectric devices made of polymeric materials are widely used
for micro- and nano-electro-mechanical systems. In particular, numerous recent
applications concern energy harvesting. Due to the importance of computational
modeling to understand the influence that microscale geometry and constitutive
variables exert on the macroscopic behavior, a numerical approach is developed
here for multiscale and multiphysics modeling of piezoelectric materials made
of aligned arrays of polymeric nanofibers. At the microscale, the
representative volume element consists in piezoelectric polymeric nanofibers,
assumed to feature a linear piezoelastic constitutive behavior and subjected to
electromechanical contact constraints using the penalty method. To avoid the
drawbacks associated with the non-smooth discretization of the master surface,
a contact smoothing approach based on B\'ezier patches is extended to the
multiphysics framework providing an improved continuity of the
parameterization. The contact element contributions to the virtual work
equations are included through suitable electric, mechanical and coupling
potentials. From the solution of the micro-scale boundary value problem, a
suitable scale transition procedure leads to the formulation of a macroscopic
thin piezoelectric shell element.Comment: 11 pages, 6 pages, 21 reference
Cooperativity in the enhanced piezoelectric response of polymer nanowires
We provide a detailed insight into piezoelectric energy generation from
arrays of polymer nanofibers. For sake of comparison, we firstly measure
individual poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFe)) fibers
at well-defined levels of compressive stress. Under an applied load of 2 mN,
single nanostructures generate a voltage of 0.45 mV. We show that under the
same load conditions, fibers in dense arrays exhibit a voltage output higher by
about two orders of magnitude. Numerical modelling studies demonstrate that the
enhancement of the piezoelectric response is a general phenomenon associated to
the electromechanical interaction among adjacent fibers, namely a cooperative
effect depending on specific geometrical parameters. This establishes new
design rules for next piezoelectric nano-generators and sensors.Comment: 31 pages, 11 figures, 1 tabl
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