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From: | Julien Bect |
Subject: | Re: nchoosek |
Date: | Mon, 01 Sep 2014 08:28:18 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux i686; rv:31.0) Gecko/20100101 Thunderbird/31.0 |
Le 01/09/2014 07:51, Daniel J Sebald a
écrit :
Julien, you mentioned commonly using nchoosek-like functions for k outside the range [0 N]. Can you give an example? Assume for a while that binocdf does not exist. Then, you could write a simple replacement as follows : binopdf = @(k) (nchoosek (n, k) * (p ^ k) * (1 - p) ^ (n - k)) and it is valid for all non-negative k. And what if k is negative? Error or 0? For negative k I can see several options... 1) Error, because selecting a negative number of objects does not make sense (whatever the size of the set). 2) Zero, because it extends the validity of the above _expression_ to all integers. 3) Negative binomial coefficient (related to the negative binomial distribution and the binomial Taylor series) I would keep the current choice (option 1) which makes more sense if you stick to the purely combinatorial interpretation of nchoosek. |
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