function gamit (a, x)
  c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
  c
  c evaluate tricomi-s incomplete gamma function defined by
  c
  c gamit = x**(-a)/gamma(a) * integral t = 0 to x of exp(-t) * t**(a-1.)
  c
  c and analytic continuation for a .le. 0.0.  gamma(x) is the complete
  c gamma function of x.  gamit is evaluated for arbitrary real values of
  c a and for non-negative values of x (even though gamit is defined for
  c x .lt. 0.0).
  c
  c      a slight deterioration of 2 or 3 digits accuracy will occur when
  c gamit is very large or very small in absolute value, because log-
  c arithmic variables are used.  also, if the parameter a is very close
  c to a negative integer (but not a negative integer), there is a loss
  c of accuracy, which is reported if the result is less than half
  c machine precision.
  c
  c ref. -- w. gautschi, an evaluation procedure for incomplete gamma
  c functions, acm trans. math. software.
  c
        external  alngam, gamr, r1mach, r9gmit, r9lgic,
       1  r9lgit
        data alneps, sqeps, bot / 3*0.0 /
  c
        if (alneps.ne.0.0) go to 10
        alneps = -alog(r1mach(3))
        sqeps = sqrt(r1mach(4))
        bot = alog(r1mach(1))
  c
   10   if (x.lt.0.0) call seteru (21hgamit   x is negative, 21, 2, 2)
  c
        if (x.ne.0.0) alx = alog(x)
        sga = 1.0
        if (a.ne.0.0) sga = sign (1.0, a)
        ainta = aint (a+0.5*sga)
        aeps = a - ainta
  c
        if (x.gt.0.0) go to 20
        gamit = 0.0
        if (ainta.gt.0.0 .or. aeps.ne.0.0) gamit = gamr(a+1.0)
        return
  c
   20   if (x.gt.1.0) go to 40
        if (a.ge.(-0.5) .or. aeps.ne.0.0) call algams (a+1.0, algap1,
       1  sgngam)
        gamit = r9gmit (a, x, algap1, sgngam, alx)
        return
  c
   40   if (a.lt.x) go to 50
        t = r9lgit (a, x, alngam(a+1.0))
        if (t.lt.bot) call erroff
        gamit = exp(t)
        return
  c
   50   alng = r9lgic (a, x, alx)
  c
  c evaluate gamit in terms of alog(gamic(a,x))
  c
        h = 1.0
        if (aeps.eq.0.0 .and. ainta.le.0.0) go to 60
        call algams (a+1.0, algap1, sgngam)
        t = alog(abs(a)) + alng - algap1
        if (t.gt.alneps) go to 70
        if (t.gt.(-alneps)) h = 1.0 - sga*sgngam*exp(t)
        if (abs(h).gt.sqeps) go to 60
        call erroff
        call seteru (32hgamit   result lt half precision, 32, 1, 1)
  c
   60   t = -a*alx + alog(abs(h))
        if (t.lt.bot) call erroff
        gamit = sign (exp(t), h)
        return
  c
   70   t = t - a*alx
        if (t.lt.bot) call erroff
        gamit = -sga*sgngam*exp(t)
        return
  c
        end