# Numbers in a circle: how many sets of consecutive numbers have positive sum?

One hundred integers are written around a circle, and it is known that their sum is $1$. We will call a subset of several successive numbers a "chain". Find the number of chains whose members have a positive sum.

I had no idea how to approach the problem, so I considered some small sets.

Consider a circle with four elements $2, -1, 1, -1$ (arranged clockwise) at positions $0, 1, 2, 3$ respectively. Clearly, this has four chains: $[0, 1], [0, 1, 2]$ (moving clockwise) and $[0, 3], [0, 3, 2]$ (moving anti-clockwise).

Now consider another circle with four elements $2, -2, -1, 2$ (arranged clockwise) at positions $0, 1, 2, 3$ respectively. This has three chains $[3, 0, 1], [3, 0]$ (moving clockwise) and $[0, 3, 2]$ (moving anticlockwise).

I don't understand this. Won't the answer be ambiguous?

• One thing that might help is that no matter how you split the circle in half (any collection of 50 and 50 will do), one half will have a positive sum and the other half will not. – Cameron Williams Jun 26 '13 at 4:43
• @CameronWilliams We can do better than that. It doesn't matter how many elements are in either set. – Calvin Lin Jun 26 '13 at 4:43
• @CalvinLin you're right. I was just giving OP an idea to break away from the wrong thought processes. – Cameron Williams Jun 26 '13 at 4:45
• You’re undercounting. Your first circle has the following seven chains (I give strings of numbers, not the positions): $2$; $2,-1$; $2,-1,1$; $2,-1,1,-1$; $1$; $1,-1,2$; and $-1,2$. Your second has these seven: $2$; $2,-2,-1,2$; $-1,2$; $-1,2,2$; $2$; $2,2$; and $2,2,-2$. – Brian M. Scott Jun 26 '13 at 4:54
• OP, you have to clarify what a chain means. @BrianM.Scott A lot of it depends on what the definition of a chain is. I think that OP considers a singleton to not be a chain. – Calvin Lin Jun 26 '13 at 4:55

## 1 Answer

Note: The question depends on your definition of a chain. If you take it to not include singletons and the full circle (as suggested by your count), then your answer will be dependent on the values you chose.

Hint: If a chain of integers have a positive sum, then the opposite set of integers have a non-positive sum.

However, the opposite set of integers may not necessarily form a chain. When does it not form a chain? This will explain why you have a discrepancy in your answer.

Hint: There are $n(n-2)$ chains in total, because the full circle is not a chain.

If we include singletons and the full circle as a chain, then the 1st hint still holds (when does the opposite net not form a chain?). There are $n(n-1) + 1$ chains (where the +1 counts the full circle just once), and hence the number of positive chains is $\frac{n(n-1)}{2} + 1$, which agrees with Brian's calculations.

• Why do you say that the full circle is not a chain? – Cameron Williams Jun 26 '13 at 4:53
• @CameronWilliams This is just based on how OP is counting. I.e. OP never included any full circle, or any singletons. This is why I changed the value. If he changes his definition (esp in light of Brian's comment), I'd update accordingly. – Calvin Lin Jun 26 '13 at 4:55