dlngam.f
1.44 KB
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double precision function dlngam (x)
c august 1980 edition. w. fullerton, c3, los alamos scientific lab.
double precision x, dxrel, pi, sinpiy, sqpi2l, sq2pil,
1 y, xmax, dint, dgamma, d9lgmc, d1mach, dlog, dsin, dsqrt
external d1mach, d9lgmc, dgamma
c
data sq2pil / 0.9189385332 0467274178 0329736405 62 d0 /
c sq2pil = alog (sqrt(2*pi)), sqpi2l = alog(sqrt(pi/2))
data sqpi2l / +.2257913526 4472743236 3097614947 441 d+0 /
data pi / 3.1415926535 8979323846 2643383279 50 d0 /
c
data xmax, dxrel / 2*0.d0 /
c
if (xmax.ne.0.d0) go to 10
xmax = d1mach(2)/dlog(d1mach(2))
dxrel = dsqrt (d1mach(4))
c
10 y = dabs (x)
if (y.gt.10.d0) go to 20
c
c dlog (dabs (dgamma(x)) ) for dabs(x) .le. 10.0
c
dlngam = dlog (dabs (dgamma(x)) )
return
c
c dlog ( dabs (dgamma(x)) ) for dabs(x) .gt. 10.0
c
20 if (y.gt.xmax) call seteru (
1 39hdlngam dabs(x) so big dlngam overflows, 39, 2, 2)
c
if (x.gt.0.d0) dlngam = sq2pil + (x-0.5d0)*dlog(x) - x + d9lgmc(y)
if (x.gt.0.d0) return
c
sinpiy = dabs (dsin(pi*y))
if (sinpiy.eq.0.d0) call seteru (
1 31hdlngam x is a negative integer, 31, 3, 2)
c
dlngam = sqpi2l + (x-0.5d0)*dlog(y) - x - dlog(sinpiy) - d9lgmc(y)
c
if (dabs((x-dint(x-0.5d0))*dlngam/x).lt.dxrel) call seteru (
1 68hdlngam answer lt half precision because x too near negative
2integer, 68, 1, 1)
return
c
end