function gami (a, x)
c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
c
c evaluate the incomplete gamma function defined by
c
c gami = integral from t = 0 to x of exp(-t) * t**(a-1.0) .
c
c gami is evaluated for positive values of a and non-negative values
c of x.  a slight deterioration of 2 or 3 digits accuracy will occur
c when gami is very large or very small, because logarithmic variables
c are used.
c
      external alngam, alog, exp, gamit
c
      if (a.le.0.0) call seteru (25hgami    a must be gt zero, 25, 1, 2)
      if (x.lt.0.0) call seteru (25hgami    x must be ge zero, 25, 2, 2)
c
      gami = 0.0
      if (x.eq.0.0) return
c
c the only error possible in the expression below is a fatal overflow.
      factor = exp (alngam(a) + a*alog(x) )
c
      gami = factor * gamit(a, x)
c
      return
      end