[Numpy-discussion] arccosh for complex numbers, goofy choice of branch

[Charles R Harris]

OK,

Which branch do we want to use. As it currently is in numpy and
scipy.special

arccosh(1.5) =  0.96242365011920694
arccosh(1.5+0j) =  -0.96242365011920705 + 0.0j

This is consistent with gsl, but inconsistent with Mathematica, NAG, Maple,
and probably all sensible implementations which use the generally accepted
principal value. I've left this inconsistency raising an error in the ufunc
tests until we make a decision. It might be nice to know what FORTRAN and
MatLab do with this.

Chuck
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