Percorrer por autor "Vieira, N."
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- Dirac’s method applied to the time-fractional diffusion-wave equationPublication . Ferreira, M.; Vieira, N.; Rodrigues, M. M.We compute the fundamental solution for time-fractional diffusion Dirac-like equations, which arise from the factorization of the multidimensional time-fractional diffusion-wave equation using Dirac’s factorization approach.
- Distributed-order relaxation-oscillation equationPublication . Rodrigues, M. M.; Ferreira, M.; Vieira, N.In this short paper, we study the Cauchy problem associated with the forced time-fractional relaxation-oscillation equation with distributed order. We employ the Laplace transform technique to derive the solution. Additionally, for the scenario without external forcing, we focus on density functions characterized by a single order, demonstrating that under these conditions, the solution can be expressed using two-parameter Mittag-Leffler functions.
- Eigenfunctions of the time‐fractional diffusion‐wave operatorPublication . Ferreira, M.; Luchko, Yu.; Rodrigues, M. M.; Vieira, N.In this paper, we present some new integral and series representations for the eigenfunctions of the multidimensional time-fractional diffusion-wave operator with the time-fractional derivative of order $\beta \in ]1,2[$ defined in the Caputo sense. The integral representations are obtained in form of the inverse Fourier-Bessel transform and as double contour integrals of the Mellin-Barnes type. Concerning series expansions, the eigenfunctions are expressed as the double generalized hypergeometric series for any $\beta \in ]1,2[$ and as Kamp\'{e} de F\'{e}riet and Lauricella series in two variables for the rational values of $\beta$. The limit cases $\beta=1$ (diffusion operator) and $\beta=2$ (wave operator) as well as an intermediate case $\beta=\frac{3}{2}$ are studied in detail. Finally, we provide several plots of the eigenfunctions to some selected eigenvalues for different particular values of the fractional derivative order $\beta$ and the spatial dimension $n$.
- Eigenfunctions of the time-fractional telegraph equation of distributed orderPublication . Vieira, N.; Rodrigues, M. M.; Ferreira, M.In this work, the eigenfunction problem for the time-fractional telegraph operator of distributed order in Rn ×R+ is considered. By employing the technique of the Fourier, Laplace and Mellin transforms, an integral representation of the eigenfunctions involving the Fox H-function is obtained.
- Factorization à la Dirac applied to the time-fractional telegraph equationPublication . Ferreira, M.; Rodrigues, M.M.; Vieira, N.This paper examines a coupled system of two-term time-fractional diffusion Dirac-type equations. The system is derived by factorizing the multi-dimensional time-fractional telegraph equation with Hilfer fractional derivatives, using the Dirac method and a triplet of Pauli matrices. Solutions are obtained using operational methods provided by the combination of the Fourier transform in the space variable and the Laplace transform in the time variable. Key results include the discovery of novel Fourier transform pairs. These pairs relate specific Fourier kernels of bivariate Mittag-Leffler functions to Fox H-functions of two variables. This allows to obtain explicit solutions of the system in both Fourier-time and space-time domains. The asymptotic behaviour of these solutions is rigorously analysed, and graphical representations are generated. Further, we show that the factorization allows for the use of alternative triplets of Pauli matrices yielding related solutions. The results obtained can be generalised to the case of 𝜓-Hilfer derivatives.
- Fractional gradient methods via ψ-Hilfer derivativePublication . Vieira, N.; Rodrigues, M. M.; Ferreira, M.Motivated by the increasing of practical applications in fractional calculus, we study the classical gradient method under the perspective of the ψ-Hilfer derivative. This allows us to cover in our study several definitions of fractional derivatives that are found in the literature. The convergence of the ψ-Hilfer continuous fractional gradient method is studied both for strongly and non-strongly convex cases. Using a series representation of the target function, we develop an algorithm for the ψ-Hilfer fractional order gradient method. The numerical method obtained by truncating higher-order terms was tested and analyzed using benchmark functions. Considering variable order differentiation and optimizing the step size, the ψ-Hilfer fractional gradient method shows better results in terms of speed and accuracy. Our results generalize previous works in the literature.
- Fundamental Solution for Natural Powers of the Fractional Laplace and Dirac Operators in the Riemann–Liouville SensePublication . Teodoro, A. Di; Ferreira, M.; Vieira, N.In this paper, we study the fundamental solution of natural powers of the n-parameter fractional Laplace and Dirac operators defined via Riemann–Liouville fractional derivatives. To do this we use iteration through the fractional Poisson equation starting from the fundamental solutions of the fractional Laplace Δa+α and Dirac Da+α operators, admitting a summable fractional derivative. The family of fundamental solutions of the corresponding natural powers of fractional Laplace and Dirac operators are expressed in operator form using the Mittag–Leffler function.
- A higher dimensional fractional Borel‐Pompeiu formula and a related hypercomplex fractional operator calculusPublication . Ferreira, M.; Kraußhar, R. S.; Rodrigues, M. M.; Vieira, N.In this paper, we develop a fractional integro-differential operator calculus for Clifford-algebra valued functions. To do that we introduce fractional analogs of the Teodorescu and Cauchy-Bitsadze operators and we investigate some of their mapping properties. As a main result, we prove a fractional Borel-Pompeiu formula based on a fractional Stokes formula. This tool in hand allows us to present a Hodge-type decomposition for the fractional Dirac operator. Our results exhibit an amazing duality relation between left and right operators and between Caputo and Riemann-Liouville fractional derivatives. We round off this paper by presenting a direct application to the resolution of boundary value problems related to Laplace operators of fractional order.
- On a fractional Sturm-Liouville problem in higher dimensionsPublication . Vieira, N.; Rodrigues, M. M.; Ferreira, M.In this short paper, we consider an n-dimensional fractional Sturm-Liouville eigenvalue problem, by using fractional versions of the gradient operator involving left Caputo and right Riemann-Liouville fractional derivatives. We study the main properties of the eigenfunctions and the eigenvalues of the associated fractional boundary problem.
- Psi-Hilfer fractional relaxation-oscillation equationPublication . Vieira, N.; Ferreira, M.; Rodrigues, M.M.In this work, we solve the ψ-Hilfer fractional relaxation-oscillation equation with a force term, where the time-fractional derivatives are in the ψ-Hilfer sense. The solution of the equation is presented in terms of bivariate Mittag-Leffler functions. An asymptotic analysis of the solution of the associated homogeneous equation is performed.