通过求解LU分解解线性方程组 大型稀疏矩阵LU分解有用的是容易计算的。不是最快的方式来计算逆矩阵，但要避免消耗储存的问题-computing selected entries of the inverse, by solving a sequence of linear equations after doing an LU factorization. Useful for large sparse matrix which LU decomposition is easy to c