Exceptions

   Exceptional Argument            Routine Behavior

   (y0(), y1(),  yn()) x < 0       Invalid argument
   (y0(), y1(),  yn()) x = 0       Overflow

   The j1() and jn()  functions can result in an underflow as x
   becomes small. The largest value of x for which this occurs is a
   function of n.

   The y1() and yn()  functions can result in an overflow as x
   becomes small. The largest value of x for which this occurs is
   a function of n.