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here is an attempt to make erf, erfc, lgamma and tgamma
accessible from math module.
erf and erfc are crude translation of the code pointed
out by Daniel Stutbach.
lgamma is also taken from sourceware.org, but doesn't
use the reentrant call for the sign.
i tested only on gcc-4.3.1/linux-2.6.26.
i'll write a testsuite soon.
some results:
Python 3.0b2 (r30b2:65080, Jul 21 2008, 13:13:13) 
[GCC 4.3.1] on linux2
>>> print(math.tgamma(0.5))
1.77245385091
>>> print(math.sqrt(math.pi))
1.77245385091
>>> print(math.tgamma(1.5))
0.886226925453
>>> print(math.sqrt(math.pi)/2)
0.886226925453
>>> print(math.sqrt(math.pi)*3/4)
1.32934038818
>>> print(math.tgamma(2.5))
1.32934038818
>>> for i in range(14):
	print(i, math.lgamma(i), math.tgamma(i))
	
0 inf inf
1 0.0 1.0
2 0.0 1.0
3 0.69314718056 2.0
4 1.79175946923 6.0
5 3.17805383035 24.0
6 4.78749174278 120.0
7 6.57925121201 720.0
8 8.52516136107 5040.0
9 10.6046029027 40320.0
10 12.8018274801 362880.0
11 15.1044125731 3628800.0
12 17.5023078459 39916800.0
13 19.9872144957 479001600.0
>>> for i in range(-14,0):
	print(i/5, math.lgamma(i/5), math.tgamma(i/5))
	
-2.8 0.129801291662 -1.13860211111
-2.6 -0.118011632805 -0.888685714647
-2.4 0.102583615968 -1.10802994703
-2.2 0.790718673676 -2.20498051842
-2.0 inf inf
-1.8 1.15942070884 3.18808591111
-1.6 0.837499812222 2.31058285808
-1.4 0.978052353322 2.65927187288
-1.2 1.57917603404 4.85095714052
-1.0 inf -inf
-0.8 1.74720737374 -5.73855464
-0.6 1.30750344147 -3.69693257293
-0.4 1.31452458994 -3.72298062203
-0.2 1.76149759083 -5.82114856863
>>> math.erf(INF)
1.0
>>> math.erf(-INF)
-1.0
>>> math.erfc(INF)
0.0
>>> math.erfc(-INF)
2.0
>>> math.erf(0.6174)
0.61741074841139409