[Rd] RFC: lchoose() vs lfactorial() etc

[Robin Hankin]

Hi Martin

I think you're absolutely right about this;
One thing I need again and again is
a multinomial function, and usually define:

 > lmultinomial
function (x)
{
    lfactorial(sum(x)) - sum(lfactorial(x))
}

 > multinomial
function (x)
{
    exp(lmultinomial(x))
}


It would be nice to have this in base R.

Is this the place to discuss having complex
arguments for gamma()?


best wishes

rksh






Martin Maechler wrote:
> lgamma(x) and lfactorial(x) are defined to return
>
>  ln|Gamma(x)| {= log(abs(gamma(x)))} or  ln|Gamma(x+1)| respectively.
>
> Unfortunately,  we haven't chosen the analogous definition for 
> lchoose().
>
> So, currently 
>
>   > lchoose(1/2, 1:10)
>    [1] -0.6931472 -2.0794415        NaN -3.2425924        NaN -3.8869494
>    [7]        NaN -4.3357508        NaN -4.6805913
>   Warning message:
>   In lchoose(n, k) : NaNs produced
>   > 
>
> which (the NaN's) is not particularly useful.
> (I have use case where I really have to workaround those NaNs.)
>
> I herebey propose to *amend* the definition of lchoose() such
> that it behaves analogously to  lgamma() and lfactorial(),
> i.e., to return
>
>    log(abs(choose(.,.))
>
> Your comments are welcome.
> Martin Maechler, ETH Zurich
>
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>   


-- 
Robin K. S. Hankin
Uncertainty Analyst
University of Cambridge
19 Silver Street
Cambridge CB3 9EP
01223-764877