4,755 research outputs found
A Virtual Element Method for elastic and inelastic problems on polytope meshes
We present a Virtual Element Method (VEM) for possibly nonlinear elastic and
inelastic problems, mainly focusing on a small deformation regime. The
numerical scheme is based on a low-order approximation of the displacement
field, as well as a suitable treatment of the displacement gradient. The
proposed method allows for general polygonal and polyhedral meshes, it is
efficient in terms of number of applications of the constitutive law, and it
can make use of any standard black-box constitutive law algorithm. Some
theoretical results have been developed for the elastic case. Several numerical
results within the 2D setting are presented, and a brief discussion on the
extension to large deformation problems is included
Virtual Elements for the Navier-Stokes problem on polygonal meshes
A family of Virtual Element Methods for the 2D Navier-Stokes equations is
proposed and analysed. The schemes provide a discrete velocity field which is
point-wise divergence-free. A rigorous error analysis is developed, showing
that the methods are stable and optimally convergent. Several numerical tests
are presented, confirming the theoretical predictions. A comparison with some
mixed finite elements is also performed
A parallel multigrid solver for multi-patch Isogeometric Analysis
Isogeometric Analysis (IgA) is a framework for setting up spline-based
discretizations of partial differential equations, which has been introduced
around a decade ago and has gained much attention since then. If large spline
degrees are considered, one obtains the approximation power of a high-order
method, but the number of degrees of freedom behaves like for a low-order
method. One important ingredient to use a discretization with large spline
degree, is a robust and preferably parallelizable solver. While numerical
evidence shows that multigrid solvers with standard smoothers (like Gauss
Seidel) does not perform well if the spline degree is increased, the multigrid
solvers proposed by the authors and their co-workers proved to behave optimal
both in the grid size and the spline degree. In the present paper, the authors
want to show that those solvers are parallelizable and that they scale well in
a parallel environment.Comment: The first author would like to thank the Austrian Science Fund (FWF)
for the financial support through the DK W1214-04, while the second author
was supported by the FWF grant NFN S117-0
Evaluating the performance of GARCH models using White´s Reality Check
time seris, GARCH models, bootstrap, reality check, volatility, financial econometrics, Monte Carlo, forecasting, riskmetrics, moving average
Criar Ciência
Antes de desenvolver o tema ―Criar ciência‖, propriamente
dito, sinto a necessidade de definir alguns termos e de enquadrar
alguns conceitos. Em primeiro lugar, este capítulo dedica-se às
Ciências ditas Exatas e Naturais, sejam elas de caráter
fundamental ou aplicado, podendo provavelmente grande parte do
que aqui está escrito aplicar-se igualmente à Tecnologia. No
entanto, será que os processos e conceitos ligados à criação da
Ciência nestas áreas do conhecimento diferem dos que regem as
Ciências Humanas e Sociais? Provavelmente não, mas cabe ao
leitor responder a esta pergunta através dos textos aqui
apresentados. Em segundo lugar, as próprias Ciências ditas Exatas
e Naturais abrangem mundos tão diversos e distintos, tanto no que
diz respeito às escalas abrangidas (espaciais ou temporais), como
ao seu próprio estado de desenvolvimento, pelo que me é
obviamente impossível abordar estas ciências no seu conjunto.
Com estas palavras exemplifico perfeitamente, o dilema da maior
parte dos cientistas: Como falar de um tema sabendo
perfeitamente que o que se irá dizer, só se verifica para alguns
casos específicos ou até mesmo para uma grande maioria, mas
nunca para tudo e todos
Three-structured smooth transition regression models based on CART algorithm
In the present work, a tree-based model that combines aspects of CART (Classification and Regression Trees) and STR (Smooth Transition Regression) is proposed. The main idea relies on specifying a parametric nonlinear model through a tree-growing procedure. The resulting model can be analysed either as a fuzzy regression or as a smooth transition regression with multiple regimes. Decisions about splits are entirely based on statistical tests of hypotheses and confidence intervals are constructed for the parameters within the terminal nodes as well as the final predictions. A Monte Carlo Experiment shows the estimators’ properties and the ability of the proposed algorithm to identify correctly several tree architectures. An application to the famous Boston Housing dataset shows that the proposed model provides better explanation with the same number of leaves as the one obtained with the CART algorithm.
Paleoenvironmental and archeological significance of alluvial placers of the Brazilian Amazon
The gold and tin-bearing alluvial placers of the Brazilian Amazon high terrains constitute a remarkable record of the environmental changes which occurred in the region during the Quaternary. The deposits are typically immature, and have been formed by mass movements under semi-arid climate. They are clearly older than the last rain forest spreading, and may be attributed to cyclic episodes which have been correlated to the Pleistocene glaciations. These interpretations are highlighted by the discovery of abundant polished stone artifacts which have been preserved within Pleistocene deposits. The archeological record allows the demarcation of an important cultural tradition, widely disseminated in the Brazilian Amazon, and gives a starting point to review our present knowledge of the Amazonian Prehistory - still centered in studies on the Várzea" region, of Holocene age. This means that the human establishment in the Amazon is much older than formerly believed
An attempt to use color as a tool for high resolution correlations between estuarine sedimentary cores from Algarve (Portugal)
Nowadays, paleoenvironmental and climatological researches focus on studying shortterm
climatic changes and one of the more sensitive environments for recording those
short-term climatic variations are the high sedimentation rate of coastal regions. Accordingly,
4 estuaries from the Algarve region, south of Portugal, have been cored
in order to study sedimentary and climatic variations during recent times (c.a. 5000
yrs). For this purpose, we applied sampling and analytical techniques from deep-sea
studies, allowing the acquirement of almost continuous data profiles, as the color data
obtained with the spectrophotometer Colortron
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