[
https://issues.apache.org/jira/browse/MATH-722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13194419#comment-13194419
]
Sébastien Brisard commented on MATH-722:
----------------------------------------
I've run some tests comparing the two formulas: I didn't notice any major
difference. Therefore I suggest to stick with the existing formula.
Meanwhile, the fix proposed by Juan must be implemented. It can be proved
mathematically that for any value of {{b}}, the value of {{tanh(a + i * b)}} is
within one ulp of 1.0 for {{abs(a) > 18.9}}, so the proposed threshold (20.0)
is fairly safe, and consistent with other parts of CM. Note that the same fix
must be implemented for {{tan}} also.
> [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
> Assignee: Sébastien Brisard
> Priority: Minor
> Labels: patch
> Fix For: 3.0
>
> 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
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
