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 compute for. Not the fastest way to compute inverse matrix, but avoid memory problem of a full matrix storage.