1.832 registros encontrados - material ARTIGO E unidade Instituto de Matemática, Estatística e Computação Científica
(1,1)-forms acting on spinors on Kähler surfaces
ARTIGO - Inglês
Assunto: Curvatura
It is known that, for Dirac operators on Riemann surfaces twisted by line bundles with Hermitian-Einstein connections, it is possible to obtain...
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It is known that, for Dirac operators on Riemann surfaces twisted by line bundles with Hermitian-Einstein connections, it is possible to obtain estimates for the first eigenvalue in terms of the topology of the twisting bundle [8]. Attempts to generalize
2D navier-stokes equation with cylindrical fractional brownian noise
ARTIGO - Inglês
Assunto: Equações diferenciais parciais estocásticas
We consider the Navier-Stokes equation on the 2D torus, with a stochastic forcing term which is a cylindrical fractional Wiener noise of Hurst...
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We consider the Navier-Stokes equation on the 2D torus, with a stochastic forcing term which is a cylindrical fractional Wiener noise of Hurst parameter H. Following Albeverio and Ferrario (Ann Probab 32(2):1632-1649, 2004) and Da Prato and Debussche (J Funct Anal 196(1):180-210, 2002) which dealt with the case H= , we prove a local existence and uniqueness result when and a global existence and uniqueness result when 1/2 < H < 1
2D poststack seismic data inversion with curvelet denoising preconditioning
ARTIGO - Inglês
Assunto: Prospecção sismica
Seismic inversion methods are highly sensitive to the noise present in the data set. The need to enhance the signal-to-noise ratio (SNR) motivates the...
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Seismic inversion methods are highly sensitive to the noise present in the data set. The need to enhance the signal-to-noise ratio (SNR) motivates the researchers do develop increasingly sophisticated denoising methods and combine them into other techniques. While some methodologies operate on a single scale, the curvelet transform established itself as multi-scale transform useful to decompose the seismic signals into multi-resolution elements. In this study, we evaluate the benefits of curvelet denoising as a preconditioning method to poststack seismic data in an 2D acoustic inversion processing using a Bayesian framework. Our tests on a synthetic data set modelled from the Marmousi model and the real data set from the Brazilian offshore Campos Basin have shown that the curvelet thresholding method can be successfully applied for random noise elimination. Even the use of a hard global threshold might allow improvements in the deepest parts. Future work will have to show whether alternatives that ensure a more robust way of selecting the coefficients can take into account the wavelength change with depth variation
3D Prestack Fourier Mixed-Domain (FMD) depth migration for VTI media with large lateral contrasts
ARTIGO - Inglês
Assunto: Análise de Fourier
Abstract: Although many 3D One-Way Wave-equation Migration (OWEM) methods exist for VTI media, most of them struggle either with the stability, the...
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Abstract: Although many 3D One-Way Wave-equation Migration (OWEM) methods exist for VTI media, most of them struggle either with the stability, the anisotropic noise or the computational cost. In this paper we present a new method based on a mixed space- and wavenumber-propagator that overcome these issues very effectively as demonstrated by the examples. The pioneering methods of phase-shift (PS) and Stolt migration in the frequency-wavenumber domain designed for laterally homogeneous media have been followed by several extensions for laterally inhomogeneous media. Referred many times to as phase-screen or generalized phase-screen methods, such extensions include as main examples of the Split-step Fourier (SSF) and the phase-shift plus interpolation (PSPI). To further refine such phase-screen techniques, we introduce a higher-order extension to SSF valid for a 3D VTI medium with large lateral contrasts in vertical velocity and anisotropy parameters. The method is denoted Fourier Mixed-Domain (FMD) prestack depth migration and can be regarded as a stable explicit algorithm. The FMD technique was tested using the 3D SEG/EAGE salt model and the 2D anisotropic Hess model with good results. Finally, FMD was applied with success to a 3D field data set from the Barents Sea including anisotropy
A backward λ-Lemma for the forward heat flow
ARTIGO - Inglês
Assunto: Difeomorfismos
The inclination or λ-lemma is a fundamental tool in finite dimensional hyperbolic dynamics. In contrast to finite dimension, we consider the forward...
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The inclination or λ-lemma is a fundamental tool in finite dimensional hyperbolic dynamics. In contrast to finite dimension, we consider the forward semi-flow on the loop space of a closed Riemannian manifold M provided by the heat flow. The main result i
DOI: https://doi.org/10.1007/s00208-014-1026-6
Texto completo: https://arxiv.org/abs/1210.3897
A basis approach to surface clustering
ARTIGO - Inglês
Assunto: Artigo de pesquisa
Abstract: This paper presents a novel method for clustering surfaces. The proposal involves first using natural splines basis functions in a tensor...
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Abstract: This paper presents a novel method for clustering surfaces. The proposal involves first using natural splines basis functions in a tensor product to smooth the data and thus reduce the dimension to a finite number of coefficients, and then using these estimated coefficients to cluster the surfaces via k-means or spectral clustering. An extension of the algorithm to clustering higher-dimensional tensors is also discussed. We show that the proposed algorithm exhibits the property of strong consistency, with or without measurement errors, in correctly clustering the data as the sample size increases. Simulation studies suggest that the proposed method outperforms the benchmark k-means and spectral algorithm which use the original data. In addition, an EGG real data example is considered to illustrate the practical application of the proposal
A bayesian approach to hybrid splines non-parametric regression
ARTIGO - Inglês
Assunto: Estatística não paramétrica
A Bayesian approach is considered to estimate the number of basis functions and the smoothing parameter of the hybrid splines non-parametric...
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A Bayesian approach is considered to estimate the number of basis functions and the smoothing parameter of the hybrid splines non-parametric regression procedure. The method used to obtain the estimate of the regression curve and its Bayesian confidence intervals is based on the reversible jump MCMC (Green 1995). Illustrations with simulated data are provided and show good performance of the proposed approach over the existing methods.
A Bayesian approach to term structure modeling using heavy-tailed distributions
ARTIGO - Inglês
Assunto: Métodos MCMC (Estatística)
In this paper, we introduce a robust extension of the three-factor model of Diebold and Li (J. Econometrics, 130: 337364, 2006) using the class of...
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In this paper, we introduce a robust extension of the three-factor model of Diebold and Li (J. Econometrics, 130: 337364, 2006) using the class of symmetric scale mixtures of normal distributions. Specific distributions examined include the multivariate normal, Student-t, slash, and variance gamma distributions. In the presence of non-normality in the data, these distributions provide an appealing robust alternative to the routine use of the normal distribution. Using a Bayesian paradigm, we developed an efficient MCMC algorithm for parameter estimation. Moreover, the mixing parameters obtained as a by-product of the scale mixture representation can be used to identify outliers. Our results reveal that the DieboldLi models based on the Student-t and slash distributions provide significant improvement in in-sample fit and out-of-sample forecast to the US yield data than the usual normal-based model. Copyright (c) 2011 John Wiley & Sons, Ltd.
A Bayesian generalized multiple group IRT model with model-fit assessment tools
ARTIGO - Inglês
Assunto: Métodos MCMC (Estatística)
The multiple group IRT model (MGM) proposed by Bock and Zimowski (1997) provides a useful framework for analyzing item response data from clustered...
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The multiple group IRT model (MGM) proposed by Bock and Zimowski (1997) provides a useful framework for analyzing item response data from clustered respondents. In the MGM, the selected groups of respondents are of specific interest such that group-specific population distributions need to be defined. The main goal is to explore the potentials of an MCMC estimation procedure and Bayesian model-fit tools for the MGM. We develop a full Gibbs sampling algorithm (FGSA) for estimation as well as a Metropolis-Hastings within Gibss sampling algorithm (MHWGS) in order to use non-conjugate priors. The FGSA is compared with Bilog-MG, which uses marginal maximum likelihood (MML) and marginal maximum a posteriori (MMAP) methods. That is; Bilog-MG provides maximum likelihood (ML) and expected a posteriori (EAP) estimates for both item and population parameters, and maximum a posteriori (MAP) estimates for the latent traits. We conclude that, in general, the results from our approach are slightly better than Bilog-MG. Besides a simultaneous MCMC estimation procedure, model-fit assessment tools are developed. Furthermore, the prior sensitivity is investigated with respect to the parameters of the latent population distributions. It will be shown that the FGSA provides a wide set of model-fit tools. The proposed model assessment tools can evaluate important model assumptions of (1) the item response function (IRF) and (2) the latent trait distributions. The utility of the proposed estimation and model-fit assessment methods will be shown using data from a longitudinal data study concerning first to fourth graders of sampled Brazilian public schools. (C) 2012 Elsevier B.V. All rights reserved.
A bi-objective multiperiod one-dimensional cutting stock problem
ARTIGO - Inglês
Assunto: Artigo original
Abstract: In this paper, we investigate the bi-objective multiperiod one-dimensional cutting stock problem that seeks to minimize the cost of...
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Abstract: In this paper, we investigate the bi-objective multiperiod one-dimensional cutting stock problem that seeks to minimize the cost of production associated with the total length of cut objects (waste) and the inventory costs related to objects and items. A mathematical model is presented and heuristically solved by a column generation method. Computational tests were performed using the Weighted Sum method, the e-Constraint method and a variation of the Benson method. The Pearson correlation coefficient was calculated in order to investigate the trade-off between the conflicting objectives of the problem. The results confirmed a strong negative correlation between the objective functions of the problem. All the applied scalar methods were able to find multiple efficient solutions for the problem in a reasonable computational time; however, the e-Constraint and the modified Benson methods performed better
ARTIGO - Inglês
Assunto: Matrizes laplacianas
In the present paper, by using the variational and topological methods, we obtain a multiplicity and bifurcation result for the following fractional...
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In the present paper, by using the variational and topological methods, we obtain a multiplicity and bifurcation result for the following fractional problems involving critical nonlinearities and a lower order perturbation:−LKv=μv+|v|2∗s−2v+g(x,v)in Ω,v=0in RN∖Ω, where Ω is an open and bounded domain with Lipschitz boundary, N>2s, with s∈(0,1), g is a lower order perturbation of the critical power |v|2∗s−2v and μ is a positive real parameter, 2∗s=2NN−2s is the fractional critical Sobolev exponent, while LK is an integro-differential operator. Precisely, we show that the number of nontrivial solutions for this equation under suitable assumptions is at least twice the multiplicity of the eigenvalue. Our conclusions improve the related results in some respects
A bivariate Birnbaum-Saunders regression model
ARTIGO - Inglês
Assunto: Distribuição de Birnbaum-Saunders
In this work, we propose a bivariate Birnbaum-Saunders regression model through the use of bivariate Sinh-normal distribution. The proposed regression...
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In this work, we propose a bivariate Birnbaum-Saunders regression model through the use of bivariate Sinh-normal distribution. The proposed regression model has its marginal as the Birnbaum-Saunders regression model of Rieck and Nedelman (1991), which has been discussed extensively by various authors with natural applications in survival and reliability studies. This bivariate regression model can be used to analyze correlated log-lifetimes of two units, in which the dependence structure between observations arises from the bivariate normal distribution. The main aim of this paper is to propose a bivariate Birnbaum-Saunders regression model and discuss some of its properties. Specifically, we have developed the moment estimation, the maximum likelihood estimation and the observed Fisher information matrix. Hypothesis testing is also performed by the use of the asymptotic normality of the maximum-likelihood estimators. Finally, the results of simulation studies as well as an application to a real data set are presented to illustrate the model and all the inferential methods developed here.
A bloch–wigner theorem over rings with many units II
ARTIGO - Inglês
Assunto: Homomorfismos (Matemática)
In this article we prove a generalization of the Bloch-Wigner exact sequence over a commutative ring with many units R. Moreover if there is a...
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In this article we prove a generalization of the Bloch-Wigner exact sequence over a commutative ring with many units R. Moreover if there is a homomorphism R -> F, F a field, such that mu(R) -> mu(F) is injective (e.g. R a domain), we generalize Suslin's Bloch Wigner exact sequence over R
A brief survey of methods for solving nonlinear least-squares problems
ARTIGO - Inglês
Assunto: Mínimos quadrados
In this paper, we present a brief survey of methods for solving nonlinear least-squares problems. We pay specific attention to methods that take into...
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In this paper, we present a brief survey of methods for solving nonlinear least-squares problems. We pay specific attention to methods that take into account the special structure of the problems. Most of the methods discussed belong to the quasi-Newton family (i.e. the structured quasi-Newton methods (SQN)). Our survey comprises some of the traditional and modern developed methods for nonlinear least-squares problems. At the end, we suggest a few topics for further research.
A broad class of discrete-time hypercomplex-valued hopfield neural networks
ARTIGO - Inglês
Assunto: Redes neurais de Hopfield
In this paper, we address the stability of a broad class of discrete-time hypercomplex-valued Hopfield-type neural networks. To ensure the neural...
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In this paper, we address the stability of a broad class of discrete-time hypercomplex-valued Hopfield-type neural networks. To ensure the neural networks belonging to this class always settle down at a stationary state, we introduce novel hypercomplex number systems referred to as real-part associative hypercomplex number systems. Real-part associative hypercomplex number systems generalize the well-known Cayley-Dickson algebras and real Clifford algebras and include the systems of real numbers, complex numbers, dual numbers, hyperbolic numbers, quaternions, tessarines, and octonions as particular instances. Apart from the novel hypercomplex number systems, we introduce a family of hypercomplex-valued activation functions called B-projection functions. Broadly speaking, a B-projection function projects the activation potential onto the set of all possible states of a hypercomplex-valued neuron. Using the theory presented in this paper, we confirm the stability analysis of several discrete-time hypercomplex-valued Hopfield-type neural networks from the literature. Moreover, we introduce and provide the stability analysis of a general class of Hopfield-type neural networks on Cayley-Dickson algebras
A Caffarelli-Kohn-Nirenberg type inequality on Riemannian manifolds
ARTIGO - Inglês
Assunto: Campos vetoriais
We establish a generalization to Riemannian manifolds of the Caffarelli-Kohn-Nirenberg inequality. The applied method is based on the use of conformal...
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We establish a generalization to Riemannian manifolds of the Caffarelli-Kohn-Nirenberg inequality. The applied method is based on the use of conformal Killing vector fields and E. Mitidieri's approach to Hardy inequalities. (C) 2010 Elsevier Ltd. All rights reserved.
A characterization of superalgebras with pseudoinvolution of exponent 2
ARTIGO - Inglês
Assunto: Lie, Superálgebras de
Let A be a superalgebra endowed with a pseudoinvolution ∗ over an algebraically closed field of characteristic zero. If A satisfies an ordinary...
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Let A be a superalgebra endowed with a pseudoinvolution ∗ over an algebraically closed field of characteristic zero. If A satisfies an ordinary non-trivial identity, then its graded ∗-codimension sequence c∗n(A), n = 1,2,…, is exponentially bounded (Ioppolo and Martino (Linear Multilinear Algebra 66(11), 2286–2304 2018). In this paper we capture this exponential growth giving a positive answer to the Amitsur’s conjecture for this kind of algebras. More precisely, we shall see that the limn→∞c∗n(A)−−−−−√n exists and it is an integer, denoted exp∗(A) and called graded ∗-exponent of A. Moreover, we shall characterize superalgebras with pseudoinvolution according to their graded ∗-exponent
A characterization of the artin–mumford curve
ARTIGO - Inglês
Assunto: Curvas algébricas
Let M be the Artin-Mumford curve over the finite prime field F-p with p > 2. By a result of Valentini and Madan, Aut(Fp) (M) congruent to H with H =...
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Let M be the Artin-Mumford curve over the finite prime field F-p with p > 2. By a result of Valentini and Madan, Aut(Fp) (M) congruent to H with H = (C-p x C-p) x Dp-1 We prove that if X is an algebraic curve of genus g = (p - 1)(2) defined over F-p such that Aut(Fp) (X) contains a subgroup isomorphic to H then X is birationally equivalent over F-p to the Artin Mumford curve M
A characterization of the set of fixed points of some smoothed operators
ARTIGO - Inglês
Assunto: Teoria do ponto fixo
Abstract: We characterize the set F of fixed points of an operator T(x) = SQ(x), where S is a positive definite, symmetric, and stochastic matrix and...
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Abstract: We characterize the set F of fixed points of an operator T(x) = SQ(x), where S is a positive definite, symmetric, and stochastic matrix and Q is a convex combination of orthogonal projections onto closed convex sets. We show that F is the set of minimizers of a convex function: the sum of a weighted average of the squares of the distances to the convex sets and a nonnegative quadratic related to the matrix S
A class of Benders decomposition methods for variational inequalities
ARTIGO - Inglês
Assunto: Desigualdades variacionais (Matemática)
We develop new variants of Benders decomposition methods for variational inequality problems. The construction is done by applying the general class...
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We develop new variants of Benders decomposition methods for variational inequality problems. The construction is done by applying the general class of Dantzig-Wolfe decomposition of Luna et al. (Math Program 143(1-2):177-209, 2014) to an appropriately defined dual of the given variational inequality, and then passing back to the primal space. As compared to previous decomposition techniques of the Benders kind for variational inequalities, the following improvements are obtained. Instead of rather specific single-valued monotone mappings, the framework includes a rather broad class of multi-valued maximally monotone ones, and single-valued nonmonotone. Subproblems' solvability is guaranteed instead of assumed, and approximations of the subproblems' mapping are allowed (which may lead, in particular, to further decomposition of subproblems, which may otherwise be not possible). In addition, with a certain suitably chosen approximation, variational inequality subproblems become simple bound-constrained optimization problems, thus easier to solve
A class of elliptic equations with singular and critical nonlinearities
ARTIGO - Inglês
Assunto: Equações diferenciais elipticas
Let Ω ⊂ RN be a bounded smooth domain, N ≥ 3, λ a positive parameter, 0 <β< 1 and 2∗ = 2N/(N − 2). We show the existence of nontrivial nonnegative...
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Let Ω ⊂ RN be a bounded smooth domain, N ≥ 3, λ a positive parameter, 0 <β< 1 and 2∗ = 2N/(N − 2). We show the existence of nontrivial nonnegative solutions of singular and critical elliptic problems −_x005F_x005F_x005F_x005F_x005F_x005F_x005F_x005F_x005F
ARTIGO - Inglês
Assunto: Espaços hiperbólicos
In this work, we develop an improved shock-capturing and high-resolution Lagrangian–Eulerian method for hyperbolic systems and balance laws. This is a...
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In this work, we develop an improved shock-capturing and high-resolution Lagrangian–Eulerian method for hyperbolic systems and balance laws. This is a new method to deal with discontinuous flux and complicated source terms having concentrations for a wide range of applications science and engineering, namely, 1D shallow-water equations, sedimentation processes, Geophysical flows in 2D, N-Wave models, and Riccati-type problems with forcing terms. We also include numerical simulations of a 1D two-phase flow model in porous media, with gravity and a nontrivial singular δ-source term representing an injection point. Moreover, we present approximate solutions for 2D nonlinear systems (Compressible Euler Flows and Shallow-Water Equations) for distinct benchmark configurations available in the literature aiming to present convincing and robust numerical results. In addition, for the linear advection model in 1D and for a smooth solution of the nonlinear Burgers’ problem, second order approximations were obtained. We also present a high-resolution approximation of the nonlinear non-convex Buckley–Leverett problem. Based on the work of A. Harten, we derive a convergent Lagrangian–Eulerian scheme that is total variation diminishing and second-order accurate, away from local extrema and discontinuous data. Additionally, using a suitable Kružkov’s entropy definition, introduced by K. H. Karlsen and J. D. Towers, we can verify that our improved Lagrangian–Eulerian scheme converges to the unique entropy solution for conservation laws with a discontinuous space-time dependent flux. A key hallmark of our method is the dynamic tracking forward of the no-flow curves, which are locally conservative and preserve the natural setting of weak entropic solutions related to hyperbolic problems that are not reversible systems in general. In the end, we have a general procedure to construct a class of Lagrangian–Eulerian schemes to deal with hyperbolic problems with or without forcing terms. The proposed scheme is free of Riemann problem solutions and no adaptive space-time discretizations are needed. The numerical experiments verify the efficiency and accuracy of our new Lagrangian–Eulerian method
A class of parabolic equations driven by the mean curvature flow
ARTIGO - Inglês
Assunto: Estabilidade
We study a class of parabolic equations which can be viewed as a generalized mean curvature flow acting on cylindrically symmetric surfaces with a...
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We study a class of parabolic equations which can be viewed as a generalized mean curvature flow acting on cylindrically symmetric surfaces with a Dirichlet condition on the boundary. We prove the existence of a unique solution by means of an approximation scheme. We also develop the theory of asymptotic stability for solutions of general parabolic problems
A Clifford bundle approach to the wave equation of a spin 1/2 fermion in the de sitter manifold
ARTIGO - Inglês
Assunto: Equação de Dirac
In this paper we give a Clifford bundle motivated approach to the wave equation of a free spin 1/2 fermion in the de Sitter manifold, a brane with...
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In this paper we give a Clifford bundle motivated approach to the wave equation of a free spin 1/2 fermion in the de Sitter manifold, a brane with topology living in the bulk spacetime and equipped with a metric field with being the inclusion map. To obtain the analog of Dirac equation in Minkowski spacetime in the structure we appropriately factorize the two Casimir invariants C (1) and C (2) of the Lie algebra of the de Sitter group using the constraint given in the linearization of C (2) as input to linearize C (1). In this way we obtain an equation that we called DHESS1, which in previous studies by other authors was simply postulated. Next we derive a wave equation (called DHESS2) for a free spin 1/2 fermion in the de Sitter manifold using a heuristic argument which is an obvious generalization of a heuristic argument (described in detail in Appendix D) permitting a derivation of the Dirac equation in Minkowski spacetime and which shows that such famous equation express nothing more than the fact that the momentum of a free particle is a constant vector field over timelike integral curves of a given velocity field. It is a remarkable fact that DHESS1 and DHESS2 coincide. One of the main ingredients in our paper is the use of the concept of Dirac-Hestenes spinor fields. Appendices B and C recall this concept and its relation with covariant Dirac spinor fields usually used by physicists