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[MATH-1283] Fixed Gamma#gamma function for values smaller than -20. Thanks to Jean Noel Delavalade

This commit is contained in:
Thomas Neidhart 2015-10-22 22:32:50 +02:00
parent 0dd621687d
commit 40f35da565
3 changed files with 19 additions and 1 deletions
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3
src/changes/changes.xml
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@ -51,6 +51,9 @@ If the output is not quite correct, check for invisible trailing spaces!
</properties>
<body>
<release version="3.6" date="XXXX-XX-XX" description="">
<action dev="tn" type="fix" issue="MATH-1283" due-to="Jean Noel Delavalade">
Fixed "Gamma#gamma(double)" for negative values smaller than -20.
</action>
<action dev="tn" type="fix" issue="MATH-1237" due-to="Ken Williams">
Fixed javadoc of methods {floorDiv,floorMod} in class "FastMath".
</action>

2
src/main/java/org/apache/commons/math3/special/Gamma.java
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@ -695,7 +695,7 @@ public class Gamma {
}
} else {
final double y = absX + LANCZOS_G + 0.5;
final double gammaAbs = SQRT_TWO_PI / x *
final double gammaAbs = SQRT_TWO_PI / absX *
FastMath.pow(y, absX + 0.5) *
FastMath.exp(-y) * lanczos(absX);
if (x > 0.0) {

15
src/test/java/org/apache/commons/math3/special/GammaTest.java
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@ -974,6 +974,21 @@ public class GammaTest {
}
}
@Test
public void testGammaNegativeDouble() {
// check that the gamma function properly switches sign
// see: https://en.wikipedia.org/wiki/Gamma_function
double previousGamma = Gamma.gamma(-18.5);
for (double x = -19.5; x > -25; x -= 1.0) {
double gamma = Gamma.gamma(x);
Assert.assertEquals( (int) FastMath.signum(previousGamma),
- (int) FastMath.signum(gamma));
previousGamma = gamma;
}
}
private void checkRelativeError(String msg, double expected, double actual,
double tolerance) {