subroutine dbesks (xnu, x, nin, bk)
c july 1980 edition.   w. fullerton, c3, los alamos scientific lab.
      double precision xnu, x, bk(1), expxi, xmax, d1mach, dexp, dlog
      external d1mach
      data xmax / 0.d0 /
c
      if (xmax.ne.0.0d0) go to 10
      xmax = -dlog(d1mach(1))
      xmax = xmax + 0.5d0*dlog(3.14d0*0.5d0/xmax)
c
 10   if (x.gt.xmax) call seteru (
     1  36hdbesks  x so big bessel k underflows, 36, 1, 2)
c
      call dbskes (xnu, x, nin, bk)
c
      expxi = dexp (-x)
      n = iabs (nin)
      do 20 i=1,n
        bk(i) = expxi * bk(i)
 20   continue
c
      return
      end