double precision function dgamic (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
      double precision a, x, aeps, ainta, algap1, alneps, alngs, alx,
     1  bot, e, eps, gstar, h, sga, sgng, sgngam, sgngs, sqeps, t,
     2  d1mach, dlngam, d9gmic, d9gmit, d9lgic, d9lgit, dint,
     3  dexp, dlog, dsqrt
      external d1mach, d9gmic, d9gmit, d9lgic, d9lgit
     1  dlngam
c
      data eps, sqeps, alneps, bot / 4*0.0d0 /
c
      if (eps.ne.0.d0) go to 10
      eps = 0.5d0*d1mach(3)
      sqeps = dsqrt (d1mach(4))
      alneps = -dlog (d1mach(3))
      bot = dlog (d1mach(1))
c
 10   if (x.lt.0.d0) call seteru (21hdgamic  x is negative, 21, 2, 2)
c
      if (x.gt.0.d0) go to 20
      if (a.le.0.d0) call seteru (
     1  47hdgamic  x = 0 and a le 0 so dgamic is undefined, 47, 3, 2)
c
      dgamic = dexp (dlngam(a+1.d0) - dlog(a))
      return
c
 20   alx = dlog (x)
      sga = 1.0d0
      if (a.ne.0.d0) sga = dsign (1.0d0, a)
      ainta = dint (a + 0.5d0*sga)
      aeps = a - ainta
c
      izero = 0
      if (x.ge.1.0d0) go to 40
c
      if (a.gt.0.5d0 .or. dabs(aeps).gt.0.001d0) go to 30
      e = 2.0d0
      if (-ainta.gt.1.d0) e = 2.d0*(-ainta+2.d0)/(ainta*ainta-1.0d0)
      e = e - alx * x**(-0.001d0)
      if (e*dabs(aeps).gt.eps) go to 30
c
      dgamic = d9gmic (a, x, alx)
      return
c
 30   call dlgams (a+1.0d0, algap1, sgngam)
      gstar = d9gmit (a, x, algap1, sgngam, alx)
      if (gstar.eq.0.d0) izero = 1
      if (gstar.ne.0.d0) alngs = dlog (dabs(gstar))
      if (gstar.ne.0.d0) sgngs = dsign (1.0d0, gstar)
      go to 50
c
 40   if (a.lt.x) dgamic = dexp (d9lgic(a, x, alx))
      if (a.lt.x) return
c
      sgngam = 1.0d0
      algap1 = dlngam (a+1.0d0)
      sgngs = 1.0d0
      alngs = d9lgit (a, x, algap1)
c
c evaluation of dgamic(a,x) in terms of tricomi-s incomplete gamma fn.
c
 50   h = 1.d0
      if (izero.eq.1) go to 60
c
      t = a*alx + alngs
      if (t.gt.alneps) go to 70
      if (t.gt.(-alneps)) h = 1.0d0 - sgngs*dexp(t)
c
      if (dabs(h).lt.sqeps) call erroff
      if (dabs(h).lt.sqeps) call seteru (
     1  32hdgamic  result lt half precision, 32, 1, 1)
c
 60   sgng = dsign (1.0d0, h) * sga * sgngam
      t = dlog(dabs(h)) + algap1 - dlog(dabs(a))
      if (t.lt.bot) call erroff
      dgamic = sgng * dexp(t)
      return
c
 70   sgng = -sgngs * sga * sgngam
      t = t + algap1 - dlog(dabs(a))
      if (t.lt.bot) call erroff
      dgamic = sgng * dexp(t)
      return
c
      end