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Wrong pochhammer symbol for negative integers (Trac #910) #1437

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scipy-gitbot opened this Issue Apr 25, 2013 · 0 comments

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Original ticket http://projects.scipy.org/scipy/ticket/910 on 2009-04-01 by @pv, assigned to @pv.

special.poch(z,m) outputs strange results for z < 1:

>>> import scipy.special as sc
>>> sc.poch(-10, 3)
1.0
>>> sc.poch(-10, 4)
1.0

I'd have expected (-10)*(-9)*(-8) and (-10)*(-9)*(-8)*(-7), as prescribed in http://functions.wolfram.com/GammaBetaErf/Pochhammer/02/0002/

The current implementation is wrong for negative integer z:

def poch(z,m):
    return _gam(z+m) / _gam(z)

@pv pv closed this in b109a80 Sep 2, 2013

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