Re: [R] comment in src/nmath/dgamma.c

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Sun 26 Jun 2005 - 17:04:55 EST

On Sat, 25 Jun 2005, Faheem Mitha wrote:

>
> Hi,
>
> In src/nmath/dgamma.c the comment at the top says
>
> * DESCRIPTION
> *
> * Computes the density of the gamma distribution,
> *
> * 1/s (x/s)^{a-1} exp(-x/s)
> * p(x;a,s) = -----------------------
> * (a-1)!
> *
> * where `s' is the scale (= 1/lambda in other parametrizations)
> * and `a' is the shape parameter ( = alpha in other contexts).
>
> Maybe I'm missing something, but shouldn't the denominator be Gamma(a)
> (since a is not necessarily an integer)?

\Gamma(x+1) _is_ the definition of x! for x not an integer. It has not defined x! and you have not defined Gamma(x) and I don't see one is necessarily preferable to the other. The user-level documentation uses Gamma(a) but it can refer to the definition in ?gamma.

Please see the posting guide and consider which list you use: comments in the R source code are not really end-user questions and R-devel is the appropriate place to discuss the R sources.

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Received on Sun Jun 26 17:10:25 2005

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