[Octave-bug-tracker] [bug #44310] log and functions which use log return incorrect result

[Michael Godfrey]

Follow-up Comment #8, bug #44310 (project octave):

I tried looking at mappers.cc and its header .h files,
but I could not follow its logic for processing
atanh() for complex arguments. However, it appears to
me that it would be better in any case if atanh called the
appropriate libm function (atanh() or catanh()). 

Making use the the log definition is mathematically correct
but may not be the best choice for numerical accuracy.
Obviously, the glibc folks think that catanh( ) should
be evaluated differently from using clog(xp/xm)/2.0.

The fact that long double complex anl = clogl(xp/xm)/2.0;
has "always" returned 4.672911e-17-1.471128e+00i suggests
that they may consider also returning this as the answer
for double complex anl = clogl(xp/xm)/2.0 to be more accurate
and this is likely true.  So, they are not likely to agree 
to a change
in their code.

So, could a mappers.cc expert take a look at this, please?



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