对于 SPD 矩阵A，存在Cholesky 分解 或，其中L、R分别是下三角矩阵和上三角矩阵。$A=LL^T$$A=R^TR$

同样在matlab中，有一个命令R = chol(A)产生。另一个命令L = chol(A,'lower')产生。但是当我用相同的大稀疏 SPD 矩阵A来实现这两个命令时，L = chol(A,'lower')比R = chol(A)快得多，所以很神秘。为什么会这样？谢谢。$A=R^TR$$A=LL^T$

clc;clear;
n =400;
A = gallery('poisson',n);

tic
R = chol(A);%   generate the triangular matrix such that A = R'*R
toc

tic
L = chol(A,'lower');%   generate the lower matrix such that A = L*L'
toc

我已经运行了这个例子3次，数值结果如下，确实证明了我所说的上面的话（我的cpu是8GB内存和matlab 2018b）：

Elapsed time is 12.089711 seconds.
Elapsed time is 8.467380 seconds.

Elapsed time is 10.372768 seconds.
Elapsed time is 8.131158 seconds.

Elapsed time is 10.027861 seconds.
Elapsed time is 8.105706 seconds.