Inverse hyperbolic tangent elementwise. x : array_like out :
ndarray arctanh is a multivalued function: for each x there are
infinitely many numbers z such that tanh(z) = x. The convention
is to return the z whose imaginary part lies in [-pi/2, pi/2].
For real-valued input data types, arctanh always returns real
output. For each value that cannot be expressed as a real number
or infinity, it yields nan and sets the invalid floating point
error flag. For complex-valued input, arctanh is a complex
analytical function that has branch cuts [-1, -inf] and [1, inf]
and is continuous from above on the former and from below on the
latter. The inverse hyperbolic tangent is also known as atanh or
tanh^-1. |[R7]||M. Abramowitz and I.A. Stegun, “Handbook of
Mathematical Functions”, 10th printing, 1964, pp. 86.
http://www.math.sfu.ca/~cbm/aands/| |[R8]||Wikipedia, “Inverse
hyperbolic function”, http://en.wikipedia.org/wiki/Arctanh| >>>
np.arctanh([0, -0.5]) array([ 0. , -0.54930614])