function gamic (a, x)
c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
c
c evaluate the complementary incomplete gamma function
c
c gamic = integral from t = x to infinity of exp(-t) * t**(a-1.)  .
c
c gamic is evaluated for arbitrary real values of a and for non-negative
c values of x (even though gamic is defined for x .lt. 0.0), except that
c for x = 0 and a .le. 0.0, gamic is undefined.
c
c      a slight deterioration of 2 or 3 digits accuracy will occur when
c gamic 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,  r1mach, r9gmic, r9gmit, r9lgic,
     1  r9lgit
      data eps, sqeps, alneps, bot / 4*0.0 /
c
      if (eps.ne.0.) go to 10
      eps = 0.5*r1mach(3)
      sqeps = sqrt(r1mach(4))
      alneps = -alog(r1mach(3))
      bot = alog(r1mach(1))
c
 10   if (x.lt.0.0) call seteru (21hgamic   x is negative, 21, 2, 2)
c
      if (x.gt.0.0) go to 20
      if (a.le.0.0) call seteru (
     1  46hgamic   x = 0 and a le 0 so gamic is undefined, 46, 3, 2)
c
      gamic = exp (alngam(a+1.0) - alog(a))
      return
c
 20   alx = alog(x)
      sga = 1.0
      if (a.ne.0.0) sga = sign (1.0, a)
      ma = a + 0.5*sga
      aeps = a - float(ma)
c
      izero = 0
      if (x.ge.1.0) go to 60
c
      if (a.gt.0.5 .or. abs(aeps).gt.0.001) go to 50
      fm = -ma
      e = 2.0
      if (fm.gt.1.0) e = 2.0*(fm+2.0)/(fm*fm-1.0)
      e = e - alx*x**(-0.001)
      if (e*abs(aeps).gt.eps) go to 50
c
      gamic = r9gmic (a, x, alx)
      return
c
 50   call algams (a+1.0, algap1, sgngam)
      gstar = r9gmit (a, x, algap1, sgngam, alx)
      if (gstar.eq.0.0) izero = 1
      if (gstar.ne.0.0) alngs = alog (abs(gstar))
      if (gstar.ne.0.0) sgngs = sign (1.0, gstar)
      go to 70
c
 60   if (a.lt.x) gamic = exp (r9lgic(a, x, alx))
      if (a.lt.x) return
c
      sgngam = 1.0
      algap1 = alngam (a+1.0)
      sgngs = 1.0
      alngs = r9lgit (a, x, algap1)
c
c evaluation of gamic(a,x) in terms of tricomi-s incomplete gamma fn.
c
 70   h = 1.0
      if (izero.eq.1) go to 80
c
      t = a*alx + alngs
      if (t.gt.alneps) go to 90
      if (t.gt.(-alneps)) h = 1.0 - sgngs*exp(t)
c
      if (abs(h).lt.sqeps) call erroff
      if (abs(h).lt.sqeps) call seteru (
     1  32hgamic   result lt half precision, 32, 1, 1)
c
 80   sgng = sign (1.0, h) * sga * sgngam
      t = alog(abs(h)) + algap1 - alog(abs(a))
      if (t.lt.bot) call erroff
      gamic = sgng * exp(t)
      return
c
 90   sgng = -sgngs * sga * sgngam
      t = t + algap1 - alog(abs(a))
      if (t.lt.bot) call erroff
      gamic = sgng * exp(t)
      return
c
      end