double precision function d9gmit (a, x, algap1, sgngam, alx)
c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
c
c compute tricomi-s incomplete gamma function for small x.
c
      double precision a, x, algap1, sgngam, alx, ae, aeps, algs, alg2,
     1  bot, eps, fk, s, sgng2, t, te, d1mach, dlngam, dexp, dlog
      data eps, bot / 2*0.d0 /
c
      if (eps.ne.0.d0) go to 10
      eps = 0.5d0*d1mach(3)
      bot = dlog (d1mach(1))
c
 10   if (x.le.0.d0) call seteru (24hd9gmit  x should be gt 0, 24, 1, 2)
c
      ma = a + 0.5d0
      if (a.lt.0.d0) ma = a - 0.5d0
      aeps = a - dble(float(ma))
c
      ae = a
      if (a.lt.(-0.5d0)) ae = aeps
c
      t = 1.d0
      te = ae
      s = t
      do 20 k=1,200
        fk = k
        te = -x*te/fk
        t = te/(ae+fk)
        s = s + t
        if (dabs(t).lt.eps*dabs(s)) go to 30
 20   continue
      call seteru (54hd9gmit  no convergence in 200 terms of taylor-s se
     1ries, 54, 2, 2)
c
 30   if (a.ge.(-0.5d0)) algs = -algap1 + dlog(s)
      if (a.ge.(-0.5d0)) go to 60
c
      algs = -dlngam(1.d0+aeps) + dlog(s)
      s = 1.0d0
      m = -ma - 1
      if (m.eq.0) go to 50
      t = 1.0d0
      do 40 k=1,m
        t = x*t/(aeps-dble(float(m+1-k)))
        s = s + t
        if (dabs(t).lt.eps*dabs(s)) go to 50
 40   continue
c
 50   d9gmit = 0.0d0
      algs = -dble(float(ma))*dlog(x) + algs
      if (s.eq.0.d0 .or. aeps.eq.0.d0) go to 60
c
      sgng2 = sgngam * dsign (1.0d0, s)
      alg2 = -x - algap1 + dlog(dabs(s))
c
      if (alg2.gt.bot) d9gmit = sgng2 * dexp(alg2)
      if (algs.gt.bot) d9gmit = d9gmit + dexp(algs)
      return
c
 60   d9gmit = dexp (algs)
      return
c
      end