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Re: Unexpected NANs in complex arithmetic

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Op 22-06-16 om 04:48 schreef Steven D'Aprano:
> I'm doing some arithmetic on complex numbers involving INFs, and getting
> unexpected NANs.
>
> py> INF = float('inf')
> py> z = INF + 3j
> py> z
> (inf+3j)
> py> -z
> (-inf-3j)
>
> So far, nothing unexpected has occurred. But:
>
> py> -1*z  # should be the same as -z
> (-inf+nanj)

What I remember from complex numbers is that a multiplication
with a number that has |z| = 1, is equivallent with a rotation.

So you should be able to get the polar representation of this
"number", add in the angle of -1, being π, and convert back to
the cartesian representation.

I think seen this way, the nan part makes perfect sense.


Also the multiplication of a+bj with c+dj is (ac-bd)+(ad+bc)j
With your "numbers" this gives.

  (inf*(-1) - 3*0) + (inf*0 + 3*(-1))j

Again the nan part makes perfect sense.

-- 
Antoon

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Unexpected NANs in complex arithmetic Steven D'Aprano <steve@pearwood.info> - 2016-06-22 12:48 +1000
  Re: Unexpected NANs in complex arithmetic Chris Angelico <rosuav@gmail.com> - 2016-06-22 12:57 +1000
    Re: Unexpected NANs in complex arithmetic Dan Sommers <dan@tombstonezero.net> - 2016-06-22 03:54 +0000
      Re: Unexpected NANs in complex arithmetic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-22 16:17 +1000
        Re: Unexpected NANs in complex arithmetic Paul Rubin <no.email@nospam.invalid> - 2016-06-21 23:25 -0700
        Re: Unexpected NANs in complex arithmetic Chris Angelico <rosuav@gmail.com> - 2016-06-22 16:30 +1000
          Re: Unexpected NANs in complex arithmetic Dan Sommers <dan@tombstonezero.net> - 2016-06-22 22:54 +0000
    Re: Unexpected NANs in complex arithmetic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-22 16:58 +1000
      Re: Unexpected NANs in complex arithmetic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-22 17:05 +1000
  Re: Unexpected NANs in complex arithmetic Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-06-22 09:14 +0200
  Re: Unexpected NANs in complex arithmetic Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-06-22 10:18 +0100

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