Inverse hyperbolic tangent elementwise.
Parameters:  x : array_like


Returns:  out : ndarray

Notes
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 realvalued 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 complexvalued 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.
References
[14]  M. Abramowitz and I.A. Stegun, “Handbook of Mathematical Functions”, 10th printing, 1964, pp. 86. http://www.math.sfu.ca/~cbm/aands/ 
[15]  Wikipedia, “Inverse hyperbolic function”, http://en.wikipedia.org/wiki/Arctanh 
Examples
>>> np.arctanh([0, 0.5])
array([ 0. , 0.54930614])