[Scipy-tickets] [SciPy] #737: gammaln valid range is incorrect

SciPy scipy-tickets@scipy....
Sat Nov 1 12:01:05 CDT 2008


#737: gammaln valid range is incorrect
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 Reporter:  cdavid  |        Owner:  cdavid
     Type:  defect  |       Status:  new   
 Priority:  high    |    Milestone:  0.7.0 
Component:  Other   |      Version:        
 Severity:  normal  |   Resolution:        
 Keywords:          |  
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Comment (by G-J):

 The info line for gammaln reads:
 *  gammaln      -- Log of the absolute value of the gamma function.

 With this definition of gammaln, the function actually works fine,
 since np.log(abs(gamma(-0.5))) is in fact 1.2655. However, this seems
 to be an unusual definition for gammaln. What is the best way to fix
 it? Options:

 1) Keep it as it is, with gammaln(x) = ln|gamma(x)|
 2) Change it so that it returns NaN for negative values of gamma(x)
 (i.e. negative x whose floor value is odd)
 3) Change it to always give NaN for negative values of x (Matlab's
 approach)
 4) Have it return complex values for negative logarithms

 Which is best?

-- 
Ticket URL: <http://scipy.org/scipy/scipy/ticket/737#comment:4>
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