X = a numeric value between -1 and 1
ATANH returns the hyperbolic arc tangent of angle X. The result is the angle (in radians) whose hyperbolic tangent is X.
ATANH(0) = 0
ATANH(J20) = 1.127029, where J20 = 0.81
ATANH(1) = Error - ATANH, domain is -1 x 1
Excel function: ATANH