function r9lgit (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
        external r1mach
        data eps, sqeps / 2*0.0 /
  c
        if (eps.eq.0.0) eps = 0.5*r1mach(3)
        if (sqeps.eq.0.0) sqeps = sqrt(r1mach(4))
  c
        if (x.le.0.0 .or. a.lt.x) call seteru (
       1  35hr9lgit  x should be gt 0.0 and le a, 35, 2, 2)
  c
        ax = a + x
        a1x = ax + 1.0
        r = 0.0
        p = 1.0
        s = p
        do 20 k=1,200
          fk = k
          t = (a+fk)*x*(1.0+r)
          r = t/((ax+fk)*(a1x+fk)-t)
          p = r*p
          s = s + p
          if (abs(p).lt.eps*s) go to 30
   20   continue
        call seteru (57hr9lgit  no convergence in 200 terms of continued f
       1raction, 57, 3, 2)
  c
   30   hstar = 1.0 - x*s/a1x
        if (hstar.lt.sqeps) call seteru (
       1  39hr9lgit  result less than half precision, 39, 1, 1)
  c
        r9lgit = -x - algap1 - alog(hstar)
  c
        return
        end