SUBROUTINE BESY

PURPOSE
   COMPUTE THE Y BESSEL FUNCTION FOR A GIVEN ARGUMENT AND ORDER

USAGE
   CALL BESY(X,N,BY,IER)

DESCRIPTION OF PARAMETERS
   X  -THE ARGUMENT OF THE Y BESSEL FUNCTION DESIRED
   N  -THE ORDER OF THE Y BESSEL FUNCTION DESIRED
   BY -THE RESULTANT Y BESSEL FUNCTION
   IER-RESULTANT ERROR CODE WHERE
      IER=0  NO ERROR
      IER=1  N IS NEGATIVE
      IER=2  X IS NEGATIVE OR ZERO
      IER=3  BY HAS EXCEEDED MAGNITUDE OF 10**70

REMARKS
   VERY SMALL VALUES OF X MAY CAUSE THE RANGE OF THE LIBRARY
   FUNCTION ALOG TO BE EXCEEDED
   X MUST BE GREATER THAN ZERO
   N MUST BE GREATER THAN OR EQUAL TO ZERO

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   NONE

METHOD
   RECURRENCE RELATION AND POLYNOMIAL APPROXIMATION TECHNIQUE
   AS DESCRIBED BY A.J.M.HITCHCOCK,'POLYNOMIAL APPROXIMATIONS
   TO BESSEL FUNCTIONS OF ORDER ZERO AND ONE AND TO RELATED
   FUNCTIONS', M.T.A.C., V.11,1957,PP.86-88, AND G.N. WATSON,
   'A TREATISE ON THE THEORY OF BESSEL FUNCTIONS', CAMBRIDGE
   UNIVERSITY PRESS, 1958, P. 62