atan() computes the principal value of the arc tangent of x
    in the interval [-pi/2,pi/2] radians for x in the interval [-
    infinity, +infinity].

    atand() computes the principal value of the arc tangent of x in
    the interval [-90,90] degrees for x in the interval [-infinity,
    +infinity].

                           Entry-Point Names

    Generic                               Compaq
    Function   Data Type   OpenVMS        Tru64 UNIX
    Name       Required    Alpha          Alpha

    atan       S_FLOAT     math$atan_s    atanf
               T_FLOAT     math$atan_t    atan
               X_FLOAT     math$atan_x    atanl
               F_FLOAT     math$atan_f
               G_FLOAT     math$atan_g

    atand      S_FLOAT     math$atand_s   atandf
               T_FLOAT     math$atand_t   atand
               X_FLOAT     math$atand_x   atandl
               F_FLOAT     math$atand_f
               G_FLOAT     math$atand_g