[R] Implementation of gamma function for large number

[Robert A LaBudde]

At 04:53 PM 10/24/2009, R_help Help wrote:
>Hi - I ran into a problem when the argument to gamma function is
>large. Says, if I do the following:
>
> > gamma(17000)
>[1] Inf
>Warning message:
>value out of range in 'gammafn'
>
>Is there anyway to get around this or other implementation? Thank you.
>
>-rc

Try the log of the gamma function instead:

 > ? gamma
 > lgamma(17000)
[1] 148592.5
 > (17000-0.5)*log(17000)-17000+0.5*log(2*pi)
[1] 148592.5

Note that, for this size number, the first few terms of the usual 
expansions gives the answer to 7 significant figures.

I will restrain myself from asking the obvious question of what 
possible use gamma(17000) could be to you, and why a simple 
"infinity" would not work just as well. I'm sure you must have good 
reason for your request.



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Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
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