double precision function d9lgit (a, x, algap1)
c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
c
c compute the log of tricomi-s incomplete gamma function with perron-s
c continued fraction for large x and for a .ge. x.
c
      double precision a, x, algap1, ax, a1x, eps, fk, hstar, p, r, s,
     1  sqeps, t, d1mach, dlog, dsqrt
      external d1mach, dlog, dsqrt
      data eps, sqeps / 2*0.d0 /
c
      if (eps.ne.0.d0) go to 10
      eps = 0.5d0*d1mach(3)
      sqeps = dsqrt (d1mach(4))
c
 10   if (x.le.0.d0 .or. a.lt.x) call seteru (
     1  35hd9lgit  x should be gt 0.0 and le a, 35, 2, 2)
c
      ax = a + x
      a1x = ax + 1.0d0
      r = 0.d0
      p = 1.d0
      s = p
      do 20 k=1,200
        fk = k
        t = (a+fk)*x*(1.d0+r)
        r = t/((ax+fk)*(a1x+fk)-t)
        p = r*p
        s = s + p
        if (dabs(p).lt.eps*s) go to 30
 20   continue
      call seteru (57hd9lgit  no convergence in 200 terms of continued f
     1raction, 57, 3, 2)
c
 30   hstar = 1.0d0 - x*s/a1x
      if (hstar.lt.sqeps) call seteru (
     1  39hd9lgit  result less than half precision, 39, 1, 1)
c
      d9lgit = -x - algap1 - dlog(hstar)
      return
c
      end