NAME
    atanh - hyperbolic arc tangent

SYNOPSIS
    atanh(x [,acc])

TYPES
    x		real
    acc		real

    return	real

DESCRIPTION
    Calculate the hyperbolic arc tangent of x with an accuracy within eps.
    By default, eps is epsilon().

    The hyperbolic arc tangent is calculated utang the formula:

	atanh(x) = ln((1 + u) / (1 - u)) / 2

EXAMPLE
    > print atanh(-0.5), atanh(1-sqrt(2)), atanh(1-sqrt(2), 1e-5)
    ~-.54930614433405484569 ~-.44068679350977151261 -.440685272216796875

LIMITS
    -1 <= x <= 1
    acc > 0

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

SEE ALSO
    acosh, asinh, cosh, epsilon, tanh, tanh
