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

[Rik]

Follow-up Comment #20, bug #43349 (project octave):

>From 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:


So they are explicit about which side of the branch cut they use and it
depends on the value of the real part of the argument x.  My patch tests x to
see which side they are on and then adjusts the phase accordingly by flipping
the sign bit of the imaginary part (which happens to have value zero).

But, I'm still not certain that it is very portable.  I could see alternative
compilers not necessarily preserving the sign bit of x mult x.  Your solution
looks more portable.  The only thing I do see is that it essentially does
automatic narrowing of -0 to +0.  Right now I can adjust the phase if I want
by calling the asin (Complex) function directly:


octave:1> asin (complex (2,0))
ans =  1.5708 + 1.3170i
octave:2> asin (complex (2,-0))
ans =  1.5708 - 1.3170i


But, I'm not convinced that this edge case is all that important.


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