LU分解
Matlab
lu函数：y=lu(A);[l,u]=lu(A);[l,u,p]=lu(A);
gilbert strang
消元法 \(E_{32}E_{31}E_{21}A=U\)
\(A=E_{21}^{-1}E_{31}^{-1}E_{32}^{-1}U=LU\)
复杂度计算：
\(n^2+(n-1)^2+...+2^2+1^2≈n^3/3\)（https://blog.csdn.net/qq_40663003/article/details/104439103）
Permutations :row exchange 置换矩阵