R语言矩阵QR分解实例

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矩阵QR分解
A为m×n矩阵可以进行QR分解，A=QR，其中：Q'Q＝I，在R中可以用函数qr()进行
QR分解，例如：
> A=matrix(1:16,4,4)
> qr(A)
$qr
[,1] [,2] [,3] [,4]
[1,] -5.4772256 -12.7801930 -2.008316e+01 -2.738613e+01
[2,] 0.3651484 -3.2659863 -6.531973e+00 -9.797959e+00
[3,] 0.5477226 -0.3781696 2.641083e-15 2.056562e-15
[4,] 0.7302967 -0.9124744 8.583032e-01 -2.111449e-16
$rank
[1] 2
$qraux
[1] 1.182574e+00 1.156135e+00 1.513143e+00 2.111449e-16
$pivot
[1] 1 2 3 4
attr(,"class")
[1] "qr"
rank项返回矩阵的秩，qr项包含了矩阵Q和R的信息，要得到矩阵Q和R，可以用函数
qr.Q()和qr.R()作用qr()的返回结果，例如：
> qr.R(qr(A))
[,1] [,2] [,3] [,4]
[1,] -5.477226 -12.780193 -2.008316e+01 -2.738613e+01
[2,] 0.000000 -3.265986 -6.531973e+00 -9.797959e+00
[3,] 0.000000 0.000000 2.641083e-15 2.056562e-15
[4,] 0.000000 0.000000 0.000000e+00 -2.111449e-16
> qr.Q(qr(A))
[,1] [,2] [,3] [,4]
[1,] -0.1825742 -8.164966e-01 -0.4000874 -0.37407225
[2,] -0.3651484 -4.082483e-01 0.2546329 0.79697056
[3,] -0.5477226 -8.131516e-19 0.6909965 -0.47172438
[4,] -0.7302967 4.082483e-01 -0.5455419 0.04882607
> qr.Q(qr(A))%*%qr.R(qr(A))
[,1] [,2] [,3] [,4]
[1,] 1 5 9 13
[2,] 2 6 10 14
[3,] 3 7 11 15
[4,] 4 8 12 16
> t(qr.Q(qr(A)))%*%qr.Q(qr(A))
[,1] [,2] [,3] [,4]
[1,] 1.000000e+00 -1.457168e-16 -6.760001e-17 -7.659550e-17
[2,] -1.457168e-16 1.000000e+00 -4.269046e-17 7.011739e-17
[3,] -6.760001e-17 -4.269046e-17 1.000000e+00 -1.596437e-16
[4,] -7.659550e-17 7.011739e-17 -1.596437e-16 1.000000e+00
> qr.X(qr(A))
[,1] [,2] [,3] [,4]
[1,] 1 5 9 13
[2,] 2 6 10 14
[3,] 3 7 11 15
[4,] 4 8 12 16