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Wladimir Neves

Publications and source records attributed to Wladimir Neves.

At least 19 recordsLinked to original sources
arXiv2026

Wave interactions and stability of Riemann solutions for a nonautonomous Chromatography-type system of Langmuir isotherm

We investigate the wave interactions and stability of Riemann solutions for a nonautonomous chromatography-type system of Langmuir isotherm with time-dependent damping and flux. The system models two-component chromatographic separation with a time-dependent saturation capacity $n(t)$, leading to a nonautonomous hyperbolic system of balance laws. We consider a perturbed Riemann problem with piecewise constant initial data having two jump discontinuities at $x = \pm\epsilon$, and construct the global weak solution by analyzing all possible wave interactions, both classical (shock waves, rarefaction waves, contact discontinuities) and nonclassical (delta shock waves). We prove that as $\epsilon \to 0$, the solution of the perturbed Riemann problem converges to the solution of the corresponding Riemann problem in the space of Radon measures, establishing the stability of Riemann solutions under small perturbations of the initial data. To the best of our knowledge, this is the first instance of wave interaction and stability analysis for a nonautonomous chromatography-type system with time-dependent coefficients. Numerical experiments using a Lax-Friedrichs type scheme illustrate the wave interaction structure, the profiles at selected times, and the asymptotic convergence as $\epsilon \to 0$.

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arXiv2026

Existence of Complementary and Variational Weak Solutions to Obstacle Problems for a Quasilinear Wave Equation

We prove the existence of weak solutions for the one obstacle problem associated with a class of quasilinear wave equations in one space dimension, extending previous results obtained in the linear case, and we also address the two obstacles problem. In contrast with the linear setting, for both strictly quasilinear cases we obtain continuous solutions in a weak complementary sense, which moreover satisfy a weak entropy condition in the free region where the string is not in contact with the obstacles. We further show that, in both the one and two obstacle cases, these solutions are variational solutions in a hyperbolic sense without the viscosity term.

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arXiv2025

Besov mixed-Morrey spaces: On an Application to the Navier-Stokes Equations

In this paper we introduce two new classes of functional spaces, namely, Besov mixed-Morrey spaces and Fourier-Besov mixed-Morrey spaces, and then we establish some basic properties for these classes. Moreover, we explore the d-dimensional incompressible Navier-Stokes equations in this context, by mean of a Bony's paraproduct approach, in order to get the global well-posedness for small initial data. These results provides a new class of initial data with a sort of anisotropy in relation to its spatial variables or frequency variables.

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arXiv2025

An analysis of the 2-D isentropic Euler Equations for a generalized polytropic gas law

In this paper we developed an analysis of the compressible, isentropic Euler equations in two spatial dimensions for a generalized polytropic gas law. The main focus is rotational flows in the subsonic regimes, described through the framework of the Euler equations expressed in self-similar variables and pseudo-velocities. A Bernoulli type equation is derived, serving as a cornerstone for establising a self-similar system tailored to rotational flows. We also developed an Ellipticity Principle for generalized polytropic gases, which is applied twice in this paper. To the best of the authors' knowledge, both applications appear for the first time. In particular, the analysis of the potential flow problem in the pseudo-subsonic regime is nontrivial for generalized polytropic gases when gamma< 1. In this setting, refined techniques, such as the Moser iteration method combined with suitable a priori estimates, are required. In the final section, the study extends to an analysis of a perturbed model, introducing the concept of quasi-potential flows, offering insights into their behavior and implications.

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arXiv2025

Global well-posedness for the Navier-Stokes system in new critical mixed-norm Besov spaces

In this work, we proved the existence of a unique global mild solution of the d-dimensional incompressible Navier-Stokes equations, for small initial data in Besov type spaces based on mixed-Lebesgue spaces; namely, mixed-norm Besov-Lebesgue spaces and also mixed-norm Fourier-Besov-Lebesgue spaces. The main tools are the Bernstein's type inequalities, Bony's paraproduct to estimate the bilinear term and a fixed point scheme in order to get the well-posedness. Our results complement and cover previous and recents result on (Fourier-)Besov spaces and, for instance, provide a new class of initial data possibly not included in BMO^{-1}(R^3) but continuously included in \dot{B}^{-1}_{\infty,infty}(R^3).

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arXiv2024

An Eulerian-Lagrangian Formulation of the Compressible Euler Equations with Vacuum

In this paper, we present a novel Eulerian-Lagrangian formulation for the compressible isentropic Euler equations with vaccum. Using the developed Lagrangian flow map formulation, we show a short-time solution for a general pressure law. A particularly appealing feature of the approach used, it is well defined in the presence of vacuum, namely for compactly supported initial data which constitute an important problem in gas dynamics. Moreover, it does so without relying on any special symmetrization. While analogous results are well understood for incompressible fluids, the compressible setting, particularly in the presence of vacuum, remained open.

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arXiv2024

Riemann problem for a nonsymmetric Keyfitz-Kranzer and pressureless gas systems with a time-dependent Coulomb-like friction term

In this paper, we study the Riemann solutions for two systems: the nonsymmetric Keyfitz-Kranzer system and the pressureless system, both of which have a time-dependent Coulomb-like friction term. Our analysis identified two types of Riemann solutions: contact discontinuities and delta-shock solutions. We obtain generalized Rankine-Hugoniot conditions, which is the support for constructing the delta-shock solution for the nonsymmetric Keyfitz-Kranzer system with a time-dependent Coulomb-like friction term. Furthermore, we demonstrate that as the pressure tends to zero, the Riemann solutions of the nonsymmetric Keyfitz-Kranzer system converge to those of the pressureless system, with both systems incorporating a time-dependent Coulomb-like friction term.

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arXiv2024

The Hele-Shaw free boundary limit of Buckley-Leverett System

This paper proposes a new approach to solving the Buckley-Leverett System, which is to consider a compressible approximation model characterized by a stiff pressure law. Passing to the incompressible limit, the compressible model gives rise to a Hele-Shaw type free boundary limit of Buckley-Leverett System, and it is shown the existence of a weak solution of it.

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arXiv2024

On a class of nonautonomous quasilinear systems with general time-gradually-degenerate damping

In this paper, we study two systems with a time-variable coefficient and general time-gradually-degenerate damping. More explicitly, we construct the Riemann solutions to the time-variable coefficient Zeldovich approximation and time-variable coefficient pressureless gas systems both with general time-gradually-degenerate damping. Applying the method of similar variables and nonlinear viscosity, we obtain classical Riemann solutions and delta shock wave solutions.

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arXiv2022

Dirichlet Problem for Degenerate Fractional Parabolic Hyperbolic Equations

We are concerned in this paper with the degenerate fractional diffusion advection equations posed in bounded domains. Due to a suitable formulation, we show the existence of weak entropy solutions for measurable and bounded initial and Dirichlet boundary data. Moreover, we prove a $L^1-$type contraction property for weak entropy solutions obtained via parabolic perturbation. This is a weak selection principle which means that the weak entropy solutions are stable in this class.

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arXiv2022

On Fractional Benney Type Systems

This paper introduces fractional type evolutionary equations modeling the interaction between short waves and long waves. We consider a fractional Benney type system, which is given by a fractional Schrödinger equation coupled with a fractional porous medium equation. Under the assumption of weak coupling or small initial data related to the fractional Schrödinger equation, it is proved the existence of weak solutions to the Cauchy problem.

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arXiv2022

On a Characterization of the Rellich-Kondrachov Theorem on Groups

Motivated by an eigenvalue-eigenfunction problem posed in IR^n x Ω, where Ω is a probability space, we are concerned in this paper with the Sobolev space on groups. Hence it is established an equivalence between locally compact Abelian groups and the space of solutions to the associated variational problem. Then, we study some conditions which characterize in a precisely manner the Rellich-Kondrachov Theorem, the principal ingredient to solve the variational problem.

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arXiv2022

Homogenization of Schrodinger equations. Extended Effective Mass Theorems for non-crystalline matter

This paper concerns the homogenization of Schrodinger equations for non-crystalline matter, that is to say the coefficients are given by the composition of stationary functions with stochastic deformations. Two rigorous results of so-called effective mass theorems in solid state physics are obtained: a general abstract result (beyond the classical stationary ergodic setting), and one for quasi-perfect materials (i.e. the disorder in the non-crystalline matter is limited). The former relies on the double-scale limits and the wave function is spanned on the Bloch basis. Therefore, we have extended the Bloch Theory which was restrict until now to crystals (periodic setting). The second result relies on the Perturbation Theory and a special case of stochastic deformations, namely stochastic perturbation of the identity.

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arXiv2022

Solvability of the Fractional Hyperbolic Keller-Segel System

We study a new nonlocal approach to the mathematical modelling of the Chemotaxis problem, which describes the random motion of a certain population due a substance concentration. Considering the initial-boundary value problem for the fractional hyperbolic Keller-Segel model, we prove the solvability of the problem. The solvability result relies mostly on fractional calculus and kinetic formulation of scalar conservation laws.

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arXiv2021

Initial mixed-boundary value problem for anisotropic fractional degenerate parabolic equations

We consider an initial mixed-boundary value problem for anisotropic fractional type degenerate parabolic equations posed in bounded domains. Namely, we consider that the boundary of the domain splits into two parts. In one of them, we impose a Dirichlet boundary condition and in the another one a Neumann condition. Under this mixed-boundary condition, we show the existence of solutions for measurable and bounded non-negative initial data. The nonlocal anisotropic diffusion effect relies on an inverse of a s-fractional type elliptic operator, and the solvability is proved for any s in (0, 1).

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arXiv2021

Stochastic transport equations with unbounded divergence

We study in this article the existence and uniqueness of solutions to a class of stochastic transport equations with irregular coefficients and unbounded divergence. In the first result we assume the drift is $L^{2}([0,T] \times \R^{d})\cap L^{\infty}([0,T] \times \R^{d})$ and the divergence is the locally integrable. In the second result we show that the smoothing acts as a selection criterion when the drift is in $L^{2}([0,T] \times \R^{d})\cap L^{\infty}([0,T] \times \R^{d})$ without any condition on the divergence.

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