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https://issues.apache.org/jira/browse/MATH-753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13271320#comment-13271320
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Gilles commented on MATH-753:
-----------------------------
I noticed that the code contains this line:
{noformat}
private static final double DEFAULT_EPSILON = 10e-15;
{noformat}
This is an unusual way to write 1e-14. Or, more probably, 1e-15 was intended.
> Better implementation for the gamma distribution density function
> -----------------------------------------------------------------
>
> Key: MATH-753
> URL: https://issues.apache.org/jira/browse/MATH-753
> Project: Commons Math
> Issue Type: Improvement
> Affects Versions: 2.2, 3.0, 3.1
> Reporter: Francesco Strino
> Assignee: Sébastien Brisard
> Priority: Minor
> Labels: improvement, stability
> Fix For: 3.1
>
> Attachments: lanczos.patch
>
>
> 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 )*(alpha-1) - 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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