function poch1 (a, x)
  c august 1980 edition.  w. fullerton, c3, los alamos scientific lab.
  c
  c evaluate a generalization of pochhammer-s symbol for special
  c situations that require especially accurate values when x is small in
  c        poch1(a,x) = (poch(a,x)-1)/x
  c                   = (gamma(a+x)/gamma(a) - 1.0)/x .
  c this specification is particularly suited for stably computing
  c expressions such as
  c        (gamma(a+x)/gamma(a) - gamma(b+x)/gamma(b))/x
  c             = poch1(a,x) - poch1(b,x)
  c note that poch1(a,0.0) = psi(a)
  c
  c when abs(x) is so small that substantial cancellation will occur if
  c the straightforward formula is used, we  use an expansion due
  c to fields and discussed by y. l. luke, the special functions and their
  c approximations, vol. 1, academic press, 1969, page 34.
  c
  c the ratio poch(a,x) = gamma(a+x)/gamma(a) is written by luke as
  c        (a+(x-1)/2)**x * polynomial in (a+(x-1)/2)**(-2) .
  c in order to maintain significance in poch1, we write for positive a
  c        (a+(x-1)/2)**x = exp(x*alog(a+(x-1)/2)) = exp(q)
  c                       = 1.0 + q*exprel(q) .
  c likewise the polynomial is written
  c        poly = 1.0 + x*poly1(a,x) .
  c thus,
  c        poch1(a,x) = (poch(a,x) - 1) / x
  c                   = exprel(q)*(q/x + q*poly1(a,x)) + poly1(a,x)
  c
        dimension bern(9), gbern(10)
        external  cot, exprel, poch, psi, r1mach
  c
  c bern(i) is the 2*i bernoulli number divided by factorial(2*i).
        data bern( 1) /   .8333333333 3333333e-01 /
        data bern( 2) /  -.1388888888 8888889e-02 /
        data bern( 3) /   .3306878306 8783069e-04 /
        data bern( 4) /  -.8267195767 1957672e-06 /
        data bern( 5) /   .2087675698 7868099e-07 /
        data bern( 6) /  -.5284190138 6874932e-09 /
        data bern( 7) /   .1338253653 0684679e-10 /
        data bern( 8) /  -.3389680296 3225829e-12 /
        data bern( 9) /   .8586062056 2778446e-14 /
  c
        data pi / 3.1415926535 8979324 e0 /
        data sqtbig, alneps / 2*0.0 /
  c
        if (sqtbig.ne.0.0) go to 10
        sqtbig = 1.0/sqrt(24.0*r1mach(1))
        alneps = alog(r1mach(3))
  c
   10   if (x.eq.0.0) poch1 = psi(a)
        if (x.eq.0.0) return
  c
        absx = abs(x)
        absa = abs(a)
        if (absx.gt.0.1*absa) go to 70
        if (absx*alog(amax1(absa,2.0)).gt.0.1) go to 70
  c
        bp = a
        if (a.lt.(-0.5)) bp = 1.0 - a - x
        incr = 0
        if (bp.lt.10.0) incr = 11.0 - bp
        b = bp + float(incr)
  c
        var = b + 0.5*(x-1.0)
        alnvar = alog(var)
        q = x*alnvar
  c
        poly1 = 0.0
        if (var.ge.sqtbig) go to 40
        var2 = (1.0/var)**2
  c
        rho = 0.5*(x+1.0)
        gbern(1) = 1.0
        gbern(2) = -rho/12.0
        term = var2
        poly1 = gbern(2)*term
  c
        nterms = -0.5*alneps/alnvar + 1.0
        if (nterms.gt.9) call seteru (
       1  49hpoch1   nterms is too big, maybe r1mach(3) is bad, 49, 1, 2)
        if (nterms.lt.2) go to 40
  c
        do 30 k=2,nterms
          gbk = 0.0
          do 20 j=1,k
            ndx = k - j + 1
            gbk = gbk + bern(ndx)*gbern(j)
   20     continue
          gbern(k+1) = -rho*gbk/float(k)
  c
          term = term * (float(2*k-2)-x)*(float(2*k-1)-x)*var2
          poly1 = poly1 + gbern(k+1)*term
   30   continue
  c
   40   poly1 = (x-1.0)*poly1
        poch1 = exprel(q)*(alnvar + q*poly1) + poly1
  c
        if (incr.eq.0) go to 60
  c
  c we have poch1(b,x).  but bp is small, so we use backwards recursion
  c to obtain poch1(bp,x).
  c
        do 50 ii=1,incr
          i = incr - ii
          binv = 1.0/(bp+float(i))
          poch1 = (poch1-binv)/(1.0+x*binv)
   50   continue
  c
   60   if (bp.eq.a) return
  c
  c we have poch1(bp,x), but a is lt -0.5.  we therefore use a reflection
  c formula to obtain poch1(a,x).
  c
        sinpxx = sin(pi*x)/x
        sinpx2 = sin(0.5*pi*x)
        trig = sinpxx*cot(pi*b) - 2.0*sinpx2*(sinpx2/x)
  c
        poch1 = trig + (1.0 + x*trig) * poch1
        return
  c
   70   call entsrc (irold, 1)
        poch1 = poch (a, x)
        if (poch1.eq.0.0) call erroff
        call entsrc (irold2, irold)
  c
        poch1 = (poch1 - 1.0) / x
        return
  c
        end