FUNCTION BESSK1(X)
      REAL*8 Y,P1,P2,P3,P4,P5,P6,P7,
     *    Q1,Q2,Q3,Q4,Q5,Q6,Q7
      DATA P1,P2,P3,P4,P5,P6,P7/1.0D0,0.15443144D0,-0.67278579D0,
     *    -0.18156897D0,-0.1919402D-1,-0.110404D-2,-0.4686D-4/
      DATA Q1,Q2,Q3,Q4,Q5,Q6,Q7/1.25331414D0,0.23498619D0,-0.3655620D-1,
     *    0.1504268D-1,-0.780353D-2,0.325614D-2,-0.68245D-3/
      IF (X.LE.2.0) THEN
        Y=X*X/4.0
        BESSK1=(LOG(X/2.0)*BESSI1(X))+(1.0/X)*(P1+Y*(P2+
     *      Y*(P3+Y*(P4+Y*(P5+Y*(P6+Y*P7))))))
      ELSE
        Y=2.0/X
        BESSK1=(EXP(-X)/SQRT(X))*(Q1+Y*(Q2+Y*(Q3+
     *      Y*(Q4+Y*(Q5+Y*(Q6+Y*Q7))))))
      ENDIF
      RETURN
      END