# [racket] Math library kudos

You're welcome!
A user not finding a documented function is excellent feedback. It means
we need to communicate better. Do you remember how you searched for a
combinations function?
Neil ⊥
On 02/20/2013 08:45 AM, Luke Vilnis wrote:
>* Ha! Sorry for not reading the documentation more thoroughly - I hope
*>* this was at least a bit educational to someone besides me :) Fantastic
*>* library and docs, by the way.
*>*
*>* On Wed, Feb 20, 2013 at 10:38 AM, Neil Toronto <neil.toronto at gmail.com
*>* <mailto:neil.toronto at gmail.com>> wrote:
*>*
*>* On 02/20/2013 06:42 AM, Luke Vilnis wrote:
*>*
*>* No problem. They should be faster even for fairly small numbers
*>* since
*>* they usually require the evaluation of a polynomial (an
*>* approximation of
*>* (log)gamma) versus repeated multiplication/division. From memory the
*>* code should be something like:
*>*
*>* (exp (fllog-gamma (+ 1.0 n)) - (fllog-gamma (+ 1.0 r)) -
*>* (fllog-gamma (+
*>* 1.0 (- n r))))
*>*
*>* fllog-gamma should also be faster than bflog-gamma or log-gamma
*>* if you
*>* don't need arbitrary precision. You're also right that this
*>* won't always
*>* give completely exact results - the Racket manual says that the only
*>* exact values are for log gamma of 1 and 2, but this usually is not a
*>* problem.
*>*
*>* PS. It looks like Racket's math collection has a built-in
*>* log-factorial
*>* function too, to avoid all the +1's, so you could try that.
*>*
*>*
*>* There's also `fllog-binomial', which computes the log number of
*>* combinations directly. IIRC, its maximum observed error is 2 ulps.
*>*
*>* Neil ⊥
*>*
*>*
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