Hi,
besides the UI-name problem I come across two other questions.
(1)
If the numerator degrees of freedom (r1) is 1, then the density function
has a pole at x=0. The draft spec does not define the return value in
that case. For x<0 it defines that the return value is 0. For x>=0 it
defines it with a term which has a subterm x^(r1/2-1). And x^(-1/2) is
not defined for x=0.
I can set the result to 0 or set "illegal argument" or perhaps set
"infinity". Please decide.
(2)
The algorithm is not stable for large values of the degrees of freedom.
There are some critical parts in the term, for example BETA(r1/2;r2/2)
in the denominator which is near 0 then, and x^(r1/2-1) which will
overflow or underflow. I can of cause alter the way the term is
calculated, but I found, that there are always critical subterms. The
problem is, that the calculating as a whole does not abort with an
error, but returns some values which are wrong. You see that they are
wrong, if you calculate a series of values, but you do not notice it,
when you calculate a single value. I notice such problems, when I create
large tables of example values.
Therefore I would like to restrict the degrees of freedom to be lower
than 1.0E15, where I do not saw any problems. The current implementation
(=cumulative right tail) has a restriction of <1.0E10. Excel and
Gnumeric have restriction to about <1.0E10 too. Are larger values really
needed in "real live"? Please decide about a restriction.
kind regards
Regina
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