NAME
    atanh - inverse hyperbolic tangent

SYNOPSIS
    atanh(x [,eps])

TYPES
    x		real
    eps		nonzero real, defaults to epsilon()

    return	real

DESCRIPTION
    Returns the atanh of x to a multiple of eps with error less in
    absolute value than .75 * eps.

    atanh(x) = ln((1 + x)/(1 - x))/2 is the real number v for whichi
    tanh(v) = x.

EXAMPLE
    > print atanh(.5,1e-5), atanh(.5,1e-10), atanh(.5,1e-15), atanh(.5,1e-20)
    .54931 .5493061443 .549306144334055 .5493061443340548457

LIMITS
    unlike sin and cos, x must be real
    eps > 0

LIBRARY
    NUMBER *qatanh(NUMBER *x, NUMBER *eps)

SEE ALSO
    asinh, acosh, asech, acsch, acoth, epsilon