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mrdivide undocumented feature?

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PeterB
PeterB on 26 Jul 2017
Commented: Titus Edelhofer on 26 Jul 2017
I have A = [0 1 ; 3 2] and B = [1 2]. For A / B, I get [0.4 ; 1.4], which is a least squares solution. Here is my problem: where is this documented? I can't see this case covered in the documentation of the mrdivide function. The documentation does say that a least-squared solution is obtained when A is rectangular, but when A is square, all it says is: " If A is a square n-by-n matrix and B is a matrix with n columns, then x = B/A is a solution to the equation x*A = B, if it exists." Since no solution exists, shouldn't it return an error? Or at least, shouldn't the behaviour be documented?

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Answers (1)

Titus Edelhofer
Titus Edelhofer on 26 Jul 2017
Hi,
I think you might have mixed up A and B from the doc with A and x from your computation:
A/x
In this case your A corresponds to the B where x corresponds to A from doc. And yes, in this case A from doc (your x) is rectangular and B (your A) has n(=2) columns. Everything fine ...
Titus

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Titus Edelhofer
Titus Edelhofer on 26 Jul 2017
Hi Peter,
sorry, but it's still the other way round:
A = [1 2];
B = [0 1 ; 3 2];
x = B / A
now we have indeed: A is m by n with m~=n and B has n columns. Note, the doc talks about B / A, you wrote A / B.
Titus
PeterB
PeterB on 26 Jul 2017
Duh! OK, now I've got it. Thanks!
Titus Edelhofer
Titus Edelhofer on 26 Jul 2017
Your welcome, glad I could help. You may mark the question then as answered, if you like ...

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