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Suppose that you have a large supply of $3$ and $7$ cent stamps. Write a recurrence relation and initial conditions for the number $S_n$ of different ways in which n cents worth of stamps can be attached to an envelope if the order in which the stamps are attached does matter.

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What is the trouble? –  Bombyx mori Nov 12 '12 at 0:56
I know you have to set the initial conditions so that S0 = 3, and S1 = 7. But I do not know how to set up the equation after that. –  MKZ Nov 12 '12 at 0:58
Let us set up the reverse recurrence scheme: $S_{n}=S_{n-7}+S_{n-3}$. Your set up makes little sense as $0,1$ cents has no choice at all - they are impossible. –  Bombyx mori Nov 12 '12 at 1:01
Note: order does not matter. Hence e.g. $S_{10}=1 \neq 2=S_3+S_7$. –  Douglas S. Stones Nov 12 '12 at 1:03

2 Answers 2

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The ordered case, as previously noted, satisfies the recurrence $$S_{\mathrm{ord}}(n)=S_{\mathrm{ord}}(n-3)+S_{\mathrm{ord}}(n-7).$$

For the unordered case, I give the following hint:

Hint (since this is flagged as homework): We can adjust this recurrence to account for the "order is not important" by adding in a correction term to account for overcounting. So, we have $$S(n)=S(n-3)+S(n-7)-[???].$$ There's quite an elegant reason for this, and I don't want to spoil this for you.

The initial conditions are merely bookkeeping (they can be counted by hand, once you know the depth of the recurrence).

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If you want to put $n$ cents worth of stamps, you can either put one $7$ cent stamp and $n-7$ cents worth of stamps (i.e. $S_{n-7}$), provided that $n \geq 7$, or one $3$ cent stamp and $n-3$ cents worth of stamps (i.e. $S_{n-3}$), provided that $n \geq 3$. As far as initial conditions are concerned, you just have to analyse what is going on for $n < 7$, which should be fairly easy.

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So for all n>= 7, Sn = 7+S(n-7) and for all n>=3, Sn = 3+S(n-3)? How would that work for n= 4? You can't have use a 3 cent stamp and then have .01$ of another. –  MKZ Nov 12 '12 at 1:04
Actually, I did not see the part where you say that the order does not matter. However, for the ordered case, the relation would be for $n \geq 7, S_n = S_{n-7} + S_{n-3}$. –  beauby Nov 12 '12 at 1:06
Sorry, I edited the original question. Order does matter –  MKZ Nov 12 '12 at 1:08
No, $S(n) = S(n-3) + S(n-7)$ for $n\ge 7$. Smaller than that you do by hand. Note however that $S(0)=1$. And I guess this is ordered, because the choice in the recursion (take 3 or 7) fixes the value of the first stamp, etc. –  Hendrik Jan Nov 12 '12 at 1:11
Can you work out why this is true? –  MKZ Nov 12 '12 at 1:19

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