SUBROUTINE BESK

   COMPUTE THE K BESSEL FUNCTION FOR A GIVEN ARGUMENT AND ORDER

USAGE
   CALL BESK(X,N,BK,IER)

DESCRIPTION OF PARAMETERS
   X  -THE ARGUMENT OF THE K BESSEL FUNCTION DESIRED
   N  -THE ORDER OF THE K BESSEL FUNCTION DESIRED
   BK -THE RESULTANT K BESSEL FUNCTION
   IER-RESULTANT ERROR CODE WHERE
      IER=0  NO ERROR
      IER=1  N IS NEGATIVE
      IER=2  X IS ZERO OR NEGATIVE
      IER=3  X .GT. 170, MACHINE RANGE EXCEEDED
      IER=4  BK .GT. 10**70

REMARKS
   N MUST BE GREATER THAN OR EQUAL TO ZERO

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   NONE

METHOD
   COMPUTES ZERO ORDER AND FIRST ORDER BESSEL FUNCTIONS USING
   SERIES APPROXIMATIONS AND THEN COMPUTES N TH ORDER FUNCTION
   USING RECURRENCE RELATION.
   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