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Sébastien Brisard commented on MATH753:

This suits me fine. The only concern I have is that using exp(log(x)) in place of x might
incur a loss of accuracy. Maybe we should use this substitution only when it is necessary
(for large values of alpha and beta). This would require a little bit of investigation to
find the appropriate thresholds.
> Better implementation for the gamma distribution density function
> 
>
> Key: MATH753
> URL: https://issues.apache.org/jira/browse/MATH753
> Project: Commons Math
> Issue Type: Improvement
> Affects Versions: 2.2
> Reporter: Francesco Strino
> Priority: Minor
> Labels: improvement, stability
> Fix For: 2.2
>
>
> The way the density of the gamma distribution function is estimated can be improved.
> It's much more stable to calculate first the log of the density and then exponentiate,
otherwise the function returns NaN for high values of the parameters alpha and beta.
> It would be sufficient to change the public double density(double x) function at line
204 in the file org.apache.commons.math.distribution.GammaDistributionImpl as follows:
> return Math.exp(Math.log( x )*(alpha1)  Math.log(beta)*alpha  x/beta  Gamma.logGamma(alpha));
> In order to improve performance, log(beta) and Gamma.logGamma(alpha) could also be precomputed
and stored during initialization.

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