[issue1640] Enhancements for mathmodule

[Mark Dickinson]

Mark Dickinson added the comment:

George: I'm certainly still interested in having asinh, acosh and atanh in math---I'm 
not sure about anyone else, but I consider these three functions to be basic 
ingredients in any math library.  They even appear in most calculus texts.  And the 
complex versions are already in cmath.

The right thing to do would be to wrap the libm functions when they exist, but provide 
fallbacks for the platforms where they don't.  I think these functions are present on 
OS X and in the GNU libm, but they might be missing on Windows.

There's a fallback version of asinh already provided in the patch to issue #1381, which 
I believe to be reasonably sound (but since I wrote it, a second opinion would be 
good).

For acosh and atanh, you'd have to find the right way to deal with domain errors (e.g. 
acosh(0.5), atanh(2.0)) and singularities (atanh(1.0), atanh(-1.0)).  I'd suggest 
trying to follow the details in Appendix F (section F.9.2) of the C99 standard where 
possible.

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