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Order of roots obtained from function roots

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Shan  Chu
Shan Chu on 14 Jun 2018
Commented: Shan Chu on 14 Jun 2018
Dear all,
I have a funny problem. I have a third-degree polynomial f(s1) which depends on a variable 'm'. When I vary 'm', at some points, the ROOTS function returns me a different order of roots. For example,
when m=217:
s1 =
2.241355481895662 - 1.756029299682315i
1.921143944919372 + 1.773701622048629i
0.215633777118811 - 0.917762110838706i
when m=218;
s1 =
1.918712404828970 + 1.767930620926665i
2.241124388076128 - 1.749637415502876i
0.222483229595857 - 0.896179899600968i
So, do you know how to solve that problem? Thanks Best


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Accepted Answer

John D'Errico
John D'Errico on 14 Jun 2018
Roots does not assure any specific order of the returned roots, nor should it rationally do so.
Feel free to sort them in any order you desire. However, you need to recognize there is no natural ordering of a set of complex numbers. Should they be sorted in terms of increasing real part? Imaginary part? Magnitude? Angle in the complex plane?

  1 Comment

Shan  Chu
Shan Chu on 14 Jun 2018
Great suggestion!!! I use 'sort' and it works well. THanks

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