On Sun, 24 Jan 2010 16:11:03 -0500, Dave Angel <da...@ieee.org> wrote:
> I didn't think of simply summing the logs.
A couple terms of Stirling's approximation work pretty well:
def log_fact_half( N ):
"""log_fact_half( n ) returns the natural logarithm of the factorial of n/2.
n need not be an integer.
Domain: n from 0 to infinity
Range: from something around -0.12 to infinity
"""
SmallFacts = (
0.0000000000000000,
-0.1207822376352453,
0.0000000000000000,
0.2846828704729192,
0.6931471805599453,
1.2009736023470743,
1.7917594692280550,
2.4537365708424423,
3.1780538303479458,
3.9578139676187165,
4.7874917427820458,
5.6625620598571418,
6.5792512120101012,
7.5343642367587327,
8.5251613610654147,
9.5492672573009969,
10.6046029027452509,
11.6893334207972686,
12.8018274800814691,
13.9406252194037634 )
if N < 0:
raise RuntimeError, \
"Negative argument in LogHalfFact!"
if N < len( SmallFacts ):
RetVal = SmallFacts[ N ]
else:
n = 0.5 * N
RetVal = (n+0.5)*math.log(n) - n + HALFLOG2PI + 1.0/(12*n) - \
1.0/(360*n*n*n)
return RetVal
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