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https://issues.apache.org/jira/browse/MATH-722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13167071#comment-13167071
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Luc Maisonobe commented on MATH-722:
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Very good idea Sébastien. Using stable formulas according to argument size is
surely a good thing. We can also use FastMath.expm1() both in numerator and
denominator, it will improve accuracy when a is close to 0.
> [math] Complex Tanh for "big" numbers
> -------------------------------------
>
> Key: MATH-722
> URL: https://issues.apache.org/jira/browse/MATH-722
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 2.2
> Environment: I'm working with Eclipse 3.6.2 on Windows XP, but the
> bug is Enviroment independent
> Reporter: Juan Barandiaran
> Priority: Minor
> Labels: patch
> Original Estimate: 0.25h
> Remaining Estimate: 0.25h
>
> Hi,
> In Complex.java the tanh is computed with the following formula:
> tanh(a + bi) = sinh(2a)/(cosh(2a)+cos(2b)) + [sin(2b)/(cosh(2a)+cos(2b))]i
> The problem that I'm finding is that as soon as "a" is a "big" number,
> both sinh(2a) and cosh(2a) are infinity and then the method tanh returns in
> the real part NaN (infinity/infinity) when it should return 1.0.
> Wouldn't it be appropiate to add something as in the FastMath library??:
> if (real>20.0){
> return createComplex(1.0, 0.0);
> }
> if (real<-20.0){
> return createComplex(-1.0, 0.0);
> }
> Best regards,
> JBB
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