[Octave-bug-tracker] [bug #43349] asin behaves differently from Matlab for arguments larger than 1

[Rik]

Update of bug #43349 (project octave):

                  Status:               Need Info => Confirmed              

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Follow-up Comment #4:

The code in question is in liboctave/numeric/lo-mappers.cc:


Complex
asin (const Complex& x)
{
  static Complex i (0, 1);

  return -i * log (i*x + sqrt (1.0 - x*x));
}


It would be possible to put a test on the sign of the real part of x to get
the same result as Matlab.  Conceptually, this is


  return -i * log (i*x + signum (real (x)) * sqrt (1.0 - x*x));


But I'm not sure it is worth it.  sin (asin (x)) returns x in either case
which seems to be the more important thing.

According to Matlab's documentation:


On the branch cut, the values of arcsin coincide with the limit "from below"
for real arguments x > 1. The values coincide with the limit "from above" for
real x < - 1:


Not everyone gets a vote, but I see that Mathematica also returns the same
branch cut values as Matlab.

If this gets changed, then probably all of the functions in lo-mappers.cc that
involve branch cuts should be reviewed.



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