For homogeneous linear first-order differential equations, it is shown that Picard’s method of successive approximations is effective to furnish a closed-form solution even if the coefficient is an arbitrary function. firstorder first order equations Picards Picard s closedform closed form function

Para equações diferenciais lineares de primeira ordem homogêneas, é demonstrado que o método de sucessivas aproximações de Picard é eficaz para fornecer uma solução em forma fechada mesmo quando o coeficiente é uma função arbitrária. homogêneas arbitrária

The Euler method of numeric approximation for solving initial-value problems is applied for describing the motion of a particle influenced by linear velocity-dependent friction and arbitrary time-dependent driving force. By employing the formula for the sums of powers of integers, it is shown that the recurrence relation obtained from the Euler method converges to the exact closed-form solution. initialvalue initial value velocitydependent velocity dependent timedependent time force integers closedform closed form solution

The problem of a particle subject to a time-dependent driving force plus a linear velocity-dependent friction can be addressed by utilizing the unilateral Fourier transform, despite the presence of derivatives of odd and even order in the differential equation. This technique yields a system of algebraic equations that combine the Fourier sine and cosine transforms. While this method is useless for solving the homogeneous equation, it can be effectively used to obtain an integral representation of the particular solution. Remarkably, this integral representation of the particular solution is expressed in terms of the Green function. This type of exactly solvable problem is relevant for students who are studying mathematical methods applied in the fields of physics and engineering at the undergraduate level, as it can serve as a useful illustration of how unilateral Fourier transforms can be employed to solve problems and to develop an understanding of Green functions, even in introductory calculus courses. timedependent time dependent velocitydependent velocity transform equation Remarkably function level functions courses

In contrast to the nondegeneracy theorem, we present various scenarios in one-dimensional quantum mechanics that demonstrate how the Wronskian of two bound-state eigenfunctions with the same energy eigenvalue can be zero without implying that the eigenfunctions are linearly dependent. It is shown that the nondegeneracy theorem fails only when the potential makes different bound-state solutions corresponding to the same energy vanish at the singular point or region of singularity. onedimensional one dimensional boundstate bound state dependent singularity

It is presented a way to obtain the closed form for Green’s function related to the nonhomogeneous one-dimensional Helmholtz equation with homogeneous Dirichlet conditions on the boundary of the domain from its Fourier sine series representation. A closed form for the sum of the series ∑ k = 1 ∞ sin k x sin k y / ( k 2 - α 2 ) is found in the process.

Abstract It is shown that the Dirac oscillator in a (2+1)-dimensional spacetime can be interpreted as a spin-1/2 fermion embedded in a transverse homogeneous magnetic field.

Abstract In a recent paper diffused in the literature, the quantum problem of a particle in a infinite square well potential was claimed to be solved via Laplace transform. Surprisingly, the correct solutions were found for an eigenvalue problem with variable defined on a finite range. As a matter of fact, the authors have misused the Laplace transform of the time-independent Schrödinger equation, and they have also used incorrectly the inversion process. In the present paper it is pointed out carefully and clearly those mistakes that might occur with the use of inappropriate methods for a given eigenvalue problem.

Abstract The quantum problem of stationary states of a particle in a box is revisited by means of the unilateral Fourier transform. Homogeneous Dirichlet boundary conditions demand a finite Fourier sine transform which is actually the Fourier sine series.

Abstract It is presented a warning about the erroneous use of unilateral Fourier transform with nonhomogeneous Dirichlet or Neumann boundary conditions in a well-known textbook on integral transforms, and also in a few papers recently diffused in the literature.

Mostra-se que o sistema constituído por dois planos infinitos e paralelos, sendo um plano condutor e o outro um plano isolante com carga distribuída uniformemente, é o exemplo mais simples do uso do método das imagens, em constraste com o que é difundido na literatura.

It is shown that the system consisting of two infinite and parallel planes, one of them a conductor, and the other an insulator with a uniform distribution of charges, is the simplest example of the use of the method of images, in contrast to what is usually written in the literature.

Investiga-se a equação de Schrödinger unidimensional com uma classe de potenciais V(|x|) que se anulam no infinito e apresentam singularidade dominante na origem na forma α/|x|β(0 < β < 2). A hermiticidade dos operadores associados com quantidades físicas observáveis é usada para determinar as condições de contorno apropriadas. Dupla degenerescência e exclusão de soluções simétricas, consoante o valor de β, são discutidas. Soluções explícitas para o átomo de hidrogênio e o potencial de Kratzer são apresentadas.

The one-dimensional Schrodinger equation for a class of potentials V(|x|) which vanish at infinity and present dominant singularity at the origin in the form α/|x|β(0 < β < 2) is investigated. The hermiticity of the operators related to observable physical quantities is used to determinate the proper boundary conditions. Double degeneracy and exclusion of symmetric solutions, depending on the value of β, are discussed. Explicit solutions for the hydrogen atom and the Kratzer potential are presented.

Investiga-se a equação de Schrödinger unidimensional com o oscilador harmônico singular. A hermiticidade dos operadores associados com quantidades físicas observáveis é usada como critério para mostrar que o oscilador singular atrativo ou repulsivo exibe um número infinito de soluções aceitáveis, contanto que o parâmetro responsável pela singularidade seja maior que um certo valor crítico, em discordância com a literatura. O problema definido em todo o eixo exibe dupla degenerescência no caso do oscilador singular e intrusão de adicionais níveis de energia no caso do oscilador não-singular. Outrossim, mostra-se que a solução do oscilador singular não pode ser obtida a partir da solução do oscilador não-singular via teoria da perturbação.

The one-dimensional Schrödinger equation with the singular harmonic oscillator is investigated. The Hermiticity of the operators related to observable physical quantities is used as a criterion to show that the attractive or repulsive singular oscillator exhibits an infinite number of acceptable solutions provided the parameter responsible for the singularity is greater than a certain critical value, in disagreement with the literature. The problem for the whole line exhibits a two-fold degeneracy in the case of the singular oscillator, and the intrusion of additional solutions in the case of a nonsingular oscillator. Additionally, it is shown that the solution of the singular oscillator can not be obtained from the nonsingular oscillator via perturbation theory.

A equação de Klein-Gordon em uma dimensão espacial é investigada com a mais geral estrutura de Lorentz para os potenciais externos. A análise e o cálculo dos coeficientes de reflexão e transmissão para o espalhamento de partículas em um potencial quadrado, com uma mistura arbitrária de acoplamentos vetorial e escalar, revelam circunstâncias que conflitam com as previsões da mecânica quântica não-relativística. Mostra-se que tais espalhamentos anômalos são mediados por estados ligados de antipartículas, ainda que as partículas incidentes tenham baixas energias. A análise dos estados ligados também revela resultados surpreendentes, destacando-se a inibição do efeito Schiff-Snyder-Weinberg pela presença de um acoplamento escalar.

The one-dimensional Klein-Gordon equation is investigated with the most general Lorentz structure for the external potentials. The analysis and calculation of the reflection and transmission coefficients for the scattering of particles in a square potential, with an arbitrary mixing of vector and scalar couplings, reveal circumstances which conflict with the predictions from nonrelativistic quantum mechanics. It is shown that such anomalous scatterings are mediated by antiparticle bound states, even if the incident particles have low energies. The analysis of bound states also reveals surprising results, remarkable is the inhibition of the Schiff-Snyder-Weinberg effect due to the presence of a scalar coupling.

A equação de Klein-Gordon em uma dimensão espacial é investigada com a mais geral estrutura de Lorentz para os potenciais externos. A análise do espalhamento de partículas em um potencial degrau com uma mistura arbitrária de acoplamentos vetorial e escalar revela que o acoplamento escalar contribui para aumentar o limiar da energia de produção de pares. Mostra-se ainda que a produção de pares torna-se factível somente quando o acoplamento vetorial excede o acoplamento escalar. Um aparente paradoxo relacionado com a localização de uma partícula em uma região do espaço arbitrariamente pequena, devido à presença do potencial escalar, é resolvido com a introdução do conceito de comprimento de onda Compton efetivo.

The one-dimensional Klein-Gordon equation is investigated with the most general Lorentz structure for the external potentials. The analysis of the scattering of particles in a step potential with an arbitrary mixing of vector and scalar couplings reveals that the scalar coupling contributes for increasing the threshold energy for the particle-antiparticle pair production. Furthermore, it is shown that the pair production is only feasible whether the vector coupling exceeds the scalar one. An apparent paradox concerning the localization of a particle in an arbitrarily small region of space, due to the presence of the scalar coupling, is solved by introducing the concept of effective Compton wavelength.

A radiação de corpo negro é analisada em um universo com D dimensões espaciais. Usando a eletrodinâmica clássica consonante com a dimensionalidade do universo, e empregando o sistema de coordenadas hiperesféricas, tanto a densidade de energia espectral quanto a densidade de energia total são calculadas em função do número de dimensões espaciais. A lei do deslocamento de Wien e a lei de Stefan-Boltzmann são apropriadamente generalizadas.

The blackbody radiation is analyzed in a universe with D spatial dimensions. Using the classical electrodynamics suited to the a D-dimensional universe and employing hyperspherical coordinates, the spectral energy density as well as the total energy density are calculated as function of the dimensionality of the universe. The Wien's displacement law and the Stefan-Boltzmann law are properly generalized.