R语言矩阵的Choleskey分解实例

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矩阵的Choleskey分解
对于正定矩阵A，可对其进行Choleskey分解，即：A=P'P，其中P为上三角矩阵，在
R中可以用函数chol()进行Choleskey分解，例如：
> A
[,1] [,2] [,3] [,4]
[1,] 2 1 1 1
[2,] 1 2 1 1
[3,] 1 1 2 1
[4,] 1 1 1 2
> chol(A)
[,1] [,2] [,3] [,4]
[1,] 1.414214 0.7071068 0.7071068 0.7071068
[2,] 0.000000 1.2247449 0.4082483 0.4082483
[3,] 0.000000 0.0000000 1.1547005 0.2886751
[4,] 0.000000 0.0000000 0.0000000 1.1180340
> t(chol(A))%*%chol(A)
[,1] [,2] [,3] [,4]
[1,] 2 1 1 1
[2,] 1 2 1 1
[3,] 1 1 2 1
[4,] 1 1 1 2
> crossprod(chol(A),chol(A))
[,1] [,2] [,3] [,4]
[1,] 2 1 1 1
[2,] 1 2 1 1
[3,] 1 1 2 1
[4,] 1 1 1 2
若矩阵为对称正定矩阵，可以利用Choleskey分解求行列式的值，如：
> prod(diag(chol(A))^2)
[1] 5
> det(A)
[1] 5
若矩阵为对称正定矩阵，可以利用Choleskey分解求矩阵的逆，这时用函数
chol2inv()，这种用法更有效。如：
> chol2inv(chol(A))
[,1] [,2] [,3] [,4]
[1,] 0.8 -0.2 -0.2 -0.2
[2,] -0.2 0.8 -0.2 -0.2
[3,] -0.2 -0.2 0.8 -0.2
[4,] -0.2 -0.2 -0.2 0.8
> solve(A)
[,1] [,2] [,3] [,4]
[1,] 0.8 -0.2 -0.2 -0.2
[2,] -0.2 0.8 -0.2 -0.2
[3,] -0.2 -0.2 0.8 -0.2
[4,] -0.2 -0.2 -0.2 0.8