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Posted to issues@commons.apache.org by "Jean Noel Delavalade (JIRA)" <ji...@apache.org> on 2015/10/22 10:40:27 UTC

[jira] [Created] (MATH-1283) Gamma function computation

Jean Noel Delavalade created MATH-1283:
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             Summary: Gamma function computation
                 Key: MATH-1283
                 URL: https://issues.apache.org/jira/browse/MATH-1283
             Project: Commons Math
          Issue Type: Bug
    Affects Versions: 3.5, 4.0
         Environment: All
            Reporter: Jean Noel Delavalade
            Priority: Minor


In the gamma method, when handling the case "absX > 20", the computation of gammaAbs should replace "x" (see code below with x in bold) by "absX".
For large negative values of x, the function returns with the wrong sign.

final double gammaAbs = SQRT_TWO_PI / *x* *
                                     FastMath.pow(y, absX + 0.5) *
                                     FastMath.exp(-y) * lanczos(absX);



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