说明：  求解抛物型方程的交替隐方向P-R差分格式的matlab程序实现。不过大家在用的时候要用到原函数f.m和精确解函数uexact.m，应用程序的时候只要修改精确解和右端项就可以了。
(Solving Parabolic Equations PR alternating direction implicit difference scheme of matlab program. But we can use the time to use the original function fm and the exact solution function uexact.m, whenever an application to amend the exact solution and the right side of it.)