bugGNU Octave - Bugs: bug #53897, poly of conjugate complex pairs...

 
 

bug #53897: poly of conjugate complex pairs should return a real polynomial

Submitted by:  Marco Caliari <caliari>
Submitted on:  Mon 14 May 2018 09:11:56 AM UTC  
 
Category:  Octave Function Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Inaccurate Result
Status:  None Assigned to:  None
Originator Name:  Open/Closed:  Open
Release:  dev Operating System:  GNU/Linux

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Mon 14 May 2018 09:11:56 AM UTC, original submission:

If you try the following

you see that N has spurious imaginary parts (quite large, in absolute value). This should not happen, since the roots x are complex conjugate pairs. We should first detect this fact (I thought to imag (sum (sort (x))) == 0, it is probably possible to do without computations, but only with sorting and comparison). Then, either we use the formula

+verbatim
(y-x(1))*(y-conj(x(1)) = y^2-2*real(x(1))*y+abs(x(1))^2
-verbatim-

for each complex conjugate pair or we simply remove the spurious imaginary parts after the computation of the coefficients.

Marco Caliari <caliari>
Project Member

 

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