Numerical methods for heat equation with variable coefficients

In this paper, two methods are developed for linear parabolic partial differential equation with variable coefficients, which are based on rational approximation to the matrix exponential functions. These methods are L-stable, third-order accurate in space and time. In the development of these methods, second-order spatial derivatives are approximated by third-order finite-difference approximations, which give a system of ordinary differential equations whose solution satisfies a recurrence relation that leads to the development of algorithms. These algorithms are tested on heat equation with variable coefficients, subject to homogeneous and/or time-dependent boundary conditions, and no oscillations are observed in the experiments. The method is also modified for a nonlinear problem. All these methods do not require complex arithmetic, and based on partial fraction technique, which is very useful for parallel processing.     

Exact controllability of wave equations with variable coefficients coupled in parallel

In this paper, we investigate exact controllability for coupled wave equations which have variable coefficients principal part. Observability inequality is obtained by using Riemannian geometry method and Carleman estimates. Furthermore, the exact controllability result is established with Dirichlet boundary controls when T>T₀, where the lower bound T₀ for the control time T is different from that obtained in the constant coefficient principal part case. Copyright © 2010 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society     

带强迫项的变系数KdV方程的多孤立波解及其应用

本文利用改进的齐次平衡法,首先得到了带强迫项的变系数KdV方程的多孤立波解,然后借助此解得到了强迫KdV方程的多孤立波解.最后作为应用例子,利用图形分析方法分析了Rossby孤立波的相互作用,指出了影响Rossby孤立波相对幅度、相位、传播方向及平衡位置的主要原因.     

时变系数下耦合KdV和Burgers方程组的孤波解*

在双曲函数展开法和Jacobi椭圆函数展开法的基础上，应用它们的扩展形式来讨论三类时变系数下耦合KdV和Burgers方程组，获得了在不同情形下的一些孤波解，其中包括类孤立子解，类冲击波解和类三角函数周期型解．     

Numerical solution of the two-dimensional Helmholtz equation with variable coefficients by the radial integration boundary integral and integro-differential equation methods

This paper presents new formulations of the boundary–domain integral equation (BDIE) and the boundary–domain integro-differential equation (BDIDE) methods for the numerical solution of the two-dimensional Helmholtz equation with variable coefficients. When the material parameters are variable (with constant or variable wave number), a parametrix is adopted to reduce the Helmholtz equation to a BDIE or BDIDE. However, when material parameters are constant (with variable wave number), the standard fundamental solution for the Laplace equation is used in the formulation. The radial integration method is then employed to convert the domain integrals arising in both BDIE and BDIDE methods into equivalent boundary integrals. The resulting formulations lead to pure boundary integral and integro-differential equations with no domain integrals. Numerical examples are presented for several simple problems, for which exact solutions are available, to demonstrate the efficiency of the proposed methods.     

A modified wave equation with diffusion effects and its application as a smoothing scheme for seismic wave propagation simulations

We present a new modified wave equation and apply it to develop a smoothing scheme for seismic wave propagation simulations. With mathematical rigour we show that the solution of the new equation, which is derived as an analog of the advection–diffusion equation, can be obtained by the spatial convolution between a solution of the wave equation and the heat kernel and has a finite propagation speed and a diffusion effect. Using numerical experiments we show that the smoothing scheme based on the modified wave equation has the following advantages. Firstly, it preserves the characteristics of the wave equation such as wave propagation speed. Secondly, it selectively removes the short-wavelength components of the solution. Lastly, the energy decreases slowly after the short-wavelength components have been removed. Since our smoothing scheme can be implemented by adding simple correction terms to usual schemes, it can easily be applied to the seismic wave equation.