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From: "Alan B" <monguin61@yahoo.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: I want to know the number of permutations.
Date: Mon, 29 Jun 2009 15:44:02 +0000 (UTC)
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"Husam Aldahiyat" <numandina@gmail.com> wrote in message <h2alri$2o3$1@fred.mathworks.com>...
> Hello,
> Just like we do with nchoosek, I want a way to obtain the number of rows of the output of perms, without obtaining what they are.
> 
> Please help!
> 
> Specifically, I want the following:
> 
>                           length(unique(perms([ones(1,20),zeros(1,20)])))
> 
> or
> 
>                           length(perms([ones(1,20),zeros(1,20)]))/400
> 
> (I think they give the same result)
> 
> end

If you're just interested in examples similar to the one you give, look at this: http://www.research.att.com/~njas/sequences/index.html?q=2+6+20+70+252&language=english&go=Search .