说明：  一个二维静电场，电位函数为φ（x, y），将正方形分成25个正方形网格。有16个内部网格点。假定16个网格点的初值都定为零，试用超松弛法确定16个内部网格点的电位值。
(A two dimensional electrostatic field, the potential function for phi (x, y), the square into 25 square grid. There are 16 internal grid point. Assume that 16 mesh point of initial value at zero, trial overrelaxation method to determine the potential value of 16 internal grid point.)