Bug in 3.0.1's betainc function (was Re: [OctDev] More than just an inconvenience Re: But in t_cdf: sporadic zero's when "no need")

[Dave Goel]

>> [ tcdf bug reported]
>>
>>   
>
> This is an octave core function and so issues should be reported to
> bug at octave.org. That being said you should upgrade from 2.1.73 as that
> is a very old version of Octave. The latest stable release is 3.0.1.
> Also the t_cdf function had its name changed to tcdf in newer versions
> of Octave (though t_cdf is still around) for compatibility with the
> Matlab Statistics Toolbox function. Finally, I think we have to check
> the behavior against the equivalent function in Matlab for the case you
> believe is in error, but I don't have access to matlab with the
> statistics toolbox and so can't do it myself.
>

Sorry about the version.  I have now installed octave 3.0.1 (on
	windoze) 
as a test system and replicated the bug there.  CC-ing to octave-dev
as an ack. of David's message.

 (I don't have matlab, but a bug is a bug.  I understand that when one
 uses discrete lookup tables, some "errors" will always creep in, but
 we should do our best to resolve them.)

The error comes from the underlying betainc function.  That is an oct
file, so I couldn't further trace the source of the error.

Betainc function should be, an increasing function of x, but here's
the problem - 

> betainc (.75, 2047, .5)

4.437e-258

> betainc (.85, 2047, .5)

0

^^ If it returned a zero for .85, it should either do likewise for
.75, or it should return a better answer for .85.


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