Fitzhugh-Nagumo nerve conduction equation: A singular perturbation analysis

A singular perturbation theory is applied to the FitzHugh-Nagumo nerve conduction equation with quadratic nonlinearity to obtain travelling pulse profiles. Starting with the zeroth-order pulse solution, the explicit first-order correction to the propagating pulses is obtained.     

Bifurcation analysis of the travelling waveform of FitzHugh-Nagumo nerve conduction model equation

The FitzHugh-Nagumo model for travelling wave type neuron excitation is studied in detail. Carrying out a linear stability analysis near the equilibrium point, we bring out various interesting bifurcations which the system admits when a specific Z(2) symmetry is present and when it is not. Based on a center manifold reduction and normal form analysis, the Hopf normal form is deduced. The condition for the onset of limit cycle oscillations is found to agree well with the numerical results. We further demonstrate numerically that the system admits a period doubling route to chaos both in the presence as well as in the absence of constant external stimuli. (c) 1997 American Institute of Physics.     

A perturbation theory for the Korteweg-De Vries equation

By writing the perturbed Korteweg-de Vries equation (1) in operator form (2), we derive equations which are a basis for a perturbation method. In particular, in the first approximation, we obtain from them equations describing the evolution of the soliton amplitude and velocity. The present theory may be extended, also, to other nonlinear evolution equations if they are solved, without perturbation, by the inverse-problem method.     

Landau-Ginzburg-Higgs方程的微扰理论

求出了一阶和二阶近似下微扰对Landau-Ginzburg-Higgs方程孤子解的影响，即求出了孤子参数随时间的缓慢变化关系及解的一阶修正和二阶修正的具体表达式. 关键词： Landau-Ginzburg-Higgs方程 孤子 微扰     

A Hopf-like equation and perturbation theory for differential delay equations

We extend techniques developed for the study of turbulent fluid flows to the statistical study of the dynamics of differential delay equations. Because the phase spaces of differential delay equations are infinite dimensional, phase-space densities for these systems are functionals. We derive a Hopf-like functional differential equation governing the evolution of these densities. The functional differential equation is reduced to an infinite chain of linear partial differential equations using perturbation theory. A necessary condition for a measure to be invariant under the action of a nonlinear differential delay equation is given. Finally, we show that the evolution equation for the density functional is the Fourier transform of the infinite-dimensional version of the Kramers-Moyal expansion.     

Variational perturbation theory for Fokker-Planck equation with nonlinear drift

We develop a recursive method for perturbative solutions of the Fokker-Planck equation with nonlinear drift. The series expansion of the time-dependent probability density in terms of powers of the coupling constant is obtained by solving a set of first-order linear ordinary differential equations. Resumming the series in the spirit of variational perturbation theory we are able to determine the probability density for all values of the coupling constant. Comparison with numerical results shows exponential convergence with increasing order.     

A new perturbation theory for transport coefficients

The Green's function method for the calculation of orbital energies developed in the preceding paper is applied to the benzene molecule. It is confirmed that the lowest-order approximation for irreducible diagrams in our theory is equivalent to the usual self-consistent field theory. Higherorder corrections to orbital energies are calculated and theoretical and experimental results are compared.     

Localized heat perturbation in harmonic 1D crystals: Solutions for the equation of anomalous heat conduction

In this paper exact analytical solutions for the equation that describes anomalous heat propagation in a harmonic 1D lattices are obtained. Rectangular, triangular and sawtooth initial perturbations of the temperature field are considered. The solution for an initially rectangular temperature profile is investigated in detail. It is shown that the decay of the solution near the wavefront is proportional to \(1/\sqrt t \). In the center of the perturbation zone the decay is proportional to 1/t. Thus, the solution decays slower near the wavefront, leaving clearly visible peaks that can be detected experimentally.     

Local field equation forA 4-coupling in renormalized perturbation theory

For the model ofA ⁴-coupling a finite form of the local field equation is proposed and checked in renormalized perturbation theory.The research reported in this paper was supported in part by the National Science Foundation.     

A simple example of a singular perturbation expansion

For the Hamiltonian?=?? _x ² -αδ(x)+λx ² the bound-(λ>0) and resonance(λ<0) states are discussed. The perturbation expansion of the ground-state? ₀(λ) in powers ofλ is divergent for allλ. The nature of the singularity atλ=0 is investigated and it turns out, that the perturbation series is asymptotic and therefore useful even forλ<0.     

An analytical study of the space-clamped FitzHugh-Nagumo nerve equations

Interesting analytical calculations were carried out for the space-clamped FitzHugh-Nagumo nerve equations with a quadratic nonlinearity in membrane potential. The analysis surprisingly brings out both the travelling waves and solitary pulse solutions for the above equations.     

Novel perturbation expansion for the Langevin equation

We discuss the randomly driven systemdx/dt= -W(x) +f(t), wheref(t) is a Gaussian random function or stirring force withf(t)f(t)=2(t–t), andW(x) is of the formgx ¹⁺². The parameter is a measure of the nonlinearity of the equation. We show how to obtain the correlation functionsx(t)f(t)···x(t( n)) _f as a power series in. We obtain three terms in the expansion and show how to use Padé approximants to analytically continue the answer in the variable. By using scaling relations, we show how to get a uniform approximation to the equal-time correlation functions valid for allg and.     

Exact and approximate solutions to the Schrödinger equation for the harmonic oscillator with a singular perturbation

I obtain exact solutions for the quantum-mechanical harmonic oscillator with a perturbation potential which belongs to a class of polynomial functions of 1/r. I show that some of the eigenfunctions enable the calculation of expectation values in closed form and are therefore suitable trial functions for the application of the variational method to related nonsolvable problems.     

Hartree-Fock perturbation theory for systems with one-particle perturbation

The Hartree-Fock perturbation theory for theN-electron system with a one-particle perturbation is rederived using the resolvent operator formalism. It is shown that the second-order contribution to the total energy can be expressed in a compact form using a properly defined effective one-particle operator. Relations of the Hartree-Fock perturbation theory with both the many-body theory and the regular Hartree-Fock formalism are discussed.