[Octave-bug-tracker] [bug #46126] Polyfit bug: Different polynomial coefficients when using optional output

[Pantxo Diribarne]

Update of bug #46126 (project octave):

                  Status:                    None => Works For Me           

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Follow-up Comment #1:

Hi,

Looks like requesting the third coefficient changes the signification of the P
output, as written in the doc:


When the third output, MU, is present the coefficients, P, are
     associated with a polynomial in

     'XHAT = (X - MU(1)) / MU(2)'
     where MU(1) = mean (X), and MU(2) = std (X).

     This linear transformation of X improves the numerical stability of
     the fit.


So it must be used as following:


x = 1:100;
y = sin (2*pi*x/100) + 0.3*rand(1,100);
plot (x, y)
hold on 
p = polyfit (x, y, 3); % normal polynomial coeffs.
plot (x, polyval (p, x), "-k")
[p, s, mu]  = polyfit (x, y, 3); % modified polynomial coeffs.
xx = (x-mu(1))/mu(2);
plot (x, polyval (p, xx), "-m")


Can I close this report or do you think the documentation should be modified.
It was quite clear to me how to use modified polynomial coeffs though.


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