acosh() returns the hyperbolic arc cosine of x for x in the
    interval [1,+infinity]. acosh(x) = ln(x + sqrt(x**2 - 1)).

    acosh() is the inverse function of cosh().  The definition of the
    acosh() function is acosh(cosh (x)) = x.

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS        Tru64 UNIX
    Name       Required

    acosh      S_FLOAT     math$acosh_s   acoshf
               T_FLOAT     math$acosh_t   acosh
               X_FLOAT     math$acosh_x   acoshl
               F_FLOAT     math$acosh_f
               G_FLOAT     math$acosh_g