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double precision function dfac (n)
c june 1977 edition. w. fullerton, c3, los alamos scientific lab.
double precision facn(31), sq2pil, x, xmax, xmin, d9lgmc,
1 dexp, dlog
external d9lgmc
c
data facn ( 1) / +.1000000000 0000000000 0000000000 000 d+1 /
data facn ( 2) / +.1000000000 0000000000 0000000000 000 d+1 /
data facn ( 3) / +.2000000000 0000000000 0000000000 000 d+1 /
data facn ( 4) / +.6000000000 0000000000 0000000000 000 d+1 /
data facn ( 5) / +.2400000000 0000000000 0000000000 000 d+2 /
data facn ( 6) / +.1200000000 0000000000 0000000000 000 d+3 /
data facn ( 7) / +.7200000000 0000000000 0000000000 000 d+3 /
data facn ( 8) / +.5040000000 0000000000 0000000000 000 d+4 /
data facn ( 9) / +.4032000000 0000000000 0000000000 000 d+5 /
data facn ( 10) / +.3628800000 0000000000 0000000000 000 d+6 /
data facn ( 11) / +.3628800000 0000000000 0000000000 000 d+7 /
data facn ( 12) / +.3991680000 0000000000 0000000000 000 d+8 /
data facn ( 13) / +.4790016000 0000000000 0000000000 000 d+9 /
data facn ( 14) / +.6227020800 0000000000 0000000000 000 d+10 /
data facn ( 15) / +.8717829120 0000000000 0000000000 000 d+11 /
data facn ( 16) / +.1307674368 0000000000 0000000000 000 d+13 /
data facn ( 17) / +.2092278988 8000000000 0000000000 000 d+14 /
data facn ( 18) / +.3556874280 9600000000 0000000000 000 d+15 /
data facn ( 19) / +.6402373705 7280000000 0000000000 000 d+16 /
data facn ( 20) / +.1216451004 0883200000 0000000000 000 d+18 /
data facn ( 21) / +.2432902008 1766400000 0000000000 000 d+19 /
data facn ( 22) / +.5109094217 1709440000 0000000000 000 d+20 /
data facn ( 23) / +.1124000727 7776076800 0000000000 000 d+22 /
data facn ( 24) / +.2585201673 8884976640 0000000000 000 d+23 /
data facn ( 25) / +.6204484017 3323943936 0000000000 000 d+24 /
data facn ( 26) / +.1551121004 3330985984 0000000000 000 d+26 /
data facn ( 27) / +.4032914611 2660563558 4000000000 000 d+27 /
data facn ( 28) / +.1088886945 0418352160 7680000000 000 d+29 /
data facn ( 29) / +.3048883446 1171386050 1504000000 000 d+30 /
data facn ( 30) / +.8841761993 7397019545 4361600000 000 d+31 /
data facn ( 31) / +.2652528598 1219105863 6308480000 000 d+33 /
c
data sq2pil / 0.9189385332 0467274178 0329736405 62 d0 /
data nmax / 0 /
c
if (nmax.ne.0) go to 10
call d9gaml (xmin, xmax)
nmax = xmax - 1.d0
c
10 if (n.lt.0) call seteru (
1 47hdfac factorial of negative integer undefined, 47, 1, 2)
c
if (n.le.30) dfac = facn(n+1)
if (n.le.30) return
c
if (n.gt.nmax) call seteru (
1 39hdfac n so big factorial(n) overflows, 39, 2, 2)
c
x = n + 1
dfac = dexp ((x-0.5d0)*dlog(x) - x + sq2pil + d9lgmc(x) )
c
return
end
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