       Re: sum over i+j+k==n

• To: mathgroup at smc.vnet.net
• Subject: [mg84430] Re: sum over i+j+k==n
• From: Szabolcs Horvát <szhorvat at gmail.com>
• Date: Mon, 24 Dec 2007 04:43:10 -0500 (EST)
• References: <fkla8k\$3v1\$1@smc.vnet.net> <476E87EB.7020205@gmail.com>

Szabolcs Horvát wrote:
> Jack Kennedy wrote:
>> Hi All,
>> I often find myself needing to sum over triples
>> (or other tuples) like i,j,k where i+j+k=n. Is there
>> a convenient way to write this in Mathematica?
>
> You didn't mention exactly how do you wish to write n as a sum of three
> numbers.  In Mathematica 6, the Combinatorica package has
> IntegerPartitions[], which does it one way.  For example:
>
> In:= << Combinatorica`
>
> In:= IntegerPartitions[10, {3}]
> Out= {{8, 1, 1}, {7, 2, 1}, {6, 3, 1}, {6, 2, 2}, {5, 4, 1}, {5, 3,
>    2}, {4, 4, 2}, {4, 3, 3}}
>

Oops ... IntegerPartitions[] is actually a built-in function (new in Mathematica 6), and not part of Combinatorica ...

Szabolcs

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