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Please, help me out

How to evaluate the number of ordered partitions of the positive integer 5

Thanks!

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[WP link above was broken](en.wikipedia.org/wiki/Partition_(number_theory)) –  user2468 Apr 7 '11 at 16:29
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Note that the Wikipedia article on partitions (number theory) has to do with unordered partitions. If order matters, then the Wikipedia article to refer to is the one on compositions. –  hardmath Apr 7 '11 at 17:57

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up vote 8 down vote accepted

Since $5$ is a smallish number, it is reasonable to try to list all of the ordered partitions, and then count. First maybe, lest we forget, write down the trivial partition $5$. Then write down $4+1$, $1+4$. Now list all the ordered partitions with $3$ as the biggest number. This is easy, $3+2$, $2+3$, $3+1+1$, $1+3+1$, $1+1+3$. Continue. After not too long, you will have a complete list.

It so happens that for this type of problem, there is a simple general formula, which one might guess by carefully finding the number of ordered partitions of $1$, of $2$, of $3$, of $4$. And there are good ways of proving that the general formula holds. Let us deal with the case $n=5$.

Put $5$ pennies in a row, leaving a little gap between consecutive pennies. There are $4$ interpenny gaps. CHOOSE any number of these gaps ($0$, $1$, $2$, $3$, or $4$) to put a grain of rice into. Any such choice gives rise to a unique ordered partition of $5$, and all of them arise in this way. For example, the trivial partition $5$ comes from using no grain. The partition $4+1$ comes from putting a grain of rice after the $4$th penny. And so on. So there are exactly as many ordered partitions of $5$ as there are ways of choosing a SUBSET of the set of gaps. But a set of $4$ elements has $2^4$ subsets.

Or else one could attack the problem by induction. For example, let $P(n)$ be the number of ordered partitions of $n$. Now look at $P(n+1)$. Ordered partitions of $n+1$ are of two types: (i) last element $1$ and (ii) last element bigger than $1$. You should be able to see that there are $P(n)$ ordered partitions of $n+1$ of each type, meaning that $P(n+1)=2P(n)$.

But after all this fancy stuff, I would like to urge that you get your hands dirty, that you list and count the ordered partitions of $n$ for $n=1$, $2$, $3$, $4$, $5$, maybe even $6$.

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" You should be able to see that there are $P(n)$ ordered partitions of $n+1$ of each type" - I know I'm late, but could you be more specific about this? –  LTS Nov 8 '13 at 0:26
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If the last element is $1$, remove it, and you get an ordered partition of $n$. Moreover, all ordered partitions of $n$ arise in this way, for any ordered partition of $n$ can be extended to an ordered partition of $n+1$ by appending a $1$. So there are $P(n)$ ordered partitions of $n+1$ with last element $1$. Now take an ordered partition with last element $k\gt 1$. Replace it by $k-1$, and leave the rest alone. You get an ordered partition of $n$. (Cont) –  André Nicolas Nov 8 '13 at 0:37
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Moreover, all ordered partitions of $n+1$ with last element $\gt 1$ can be obtained by adding (not appending) a $1$ to the last element of an ordered partition of $n$. So the number of ordered partitions of $n+1$ with last element greater than $1$ is $P(n)$. So to count ordered partitions of $n+1$, we count the ones that end in $1$ ($P(n)$) and the ones that don't (another $P(n)$) for a total of $2P(n)$. We double each time. –  André Nicolas Nov 8 '13 at 0:41

So $4+1$ is one example. $2+2+1$ is another

  • What kinds of things add up to 5? (only numbers greater than or equal to 1 are used).

  • What's the least number of numbers you can use? What's the greatest number?

  • What if you rearrange the order of something you already have? Do you get something new (if you consider at as ordered)?

  • Have you done it already for 1,2,3, and 4? You might be able to use those to help with 5.

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