说明：  数值求解正方形域上的Poisson方程边值问题，用由椭圆型第一边值问题的五点差分格式，用Gauss-Seidel迭代法、块Gauss-seidel迭代法、SSOR迭代法编写求解线性方程组Au=f的算法程序
(Numerical Solution of the Poisson equation on a square domain boundary value problem, with the first boundary value problem by the oval five-point difference scheme, with the SSOR iterative method for solving linear equations written Au = f in the algorithm procedure)