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From "Luc Maisonobe (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MATH-722) [math] Complex Tanh for "big" numbers
Date Sun, 11 Dec 2011 09:19:40 GMT

    [ https://issues.apache.org/jira/browse/MATH-722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13167071#comment-13167071
] 

Luc Maisonobe commented on MATH-722:
------------------------------------

Very good idea S├ębastien. 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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