What is the different between "null" command and "svd" command

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yang-En Hsiao 2019 年 9 月 24 日

If there is a 5 by 3 matrix H ,i can use SVD to find the null space of H.But i find there is a "null "command,which can help me to find Null space of matrix.
At first ,i thought the result of using SVD command and null command will be the same,however,it is not.
For example
%Produce a 5 by 3 matrix
H = randn(5,3)
%using null command
nuA=null(H)
%Using SVD to find the null space basis ,null space will be in UA
[UA,SA,VA] = svd(H)
svA=UA
svddA=svA((1:5),4)+svA((1:5),5)
%we can find the result of svddA multiple H is almost zero
see=svddA'*H
The window show me svddA is a 5 by 1 matrix,however.
nuA show me Empty matrix: 3-by-0
Why?what is different between them?

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回答 (1 件)

Bruno Luong 2019 年 9 月 24 日
Beside the trivial must do "reading the doc" you can just type
edit null
to see how NULL call SVD and keeps the singular vectors of the NULL space.

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