### Putnam 1996

Problem B5

We call a finite string of the symbols X and O balanced iff every substring of consecutive symbols has a difference of at most 2 between the number of Xs and the number of Os. For example, XOOXOOX is not balanced, because the substring OOXOO has a difference of 3. Find the number of balanced strings of length n.

Solution

Answer: 3.2n/2 - 2 for n even, 2[n/2]+2 - 2 for n odd.

Easy if you have the right insight. Moderately difficult otherwise (analyze cases, find a recurrence relation and solve it).

The insight is to change focus from the identity of the symbol to the number of consecutive symbols of a given type. We cannot have XXX or OOO, so we have blocks of length 1 or 2, alternately X and O. If we code the string as 11122112... , then each such code corresponds to two possible strings depending on whether we start with X or O.

The balance requirement translates to requiring an even number of 1s between each pair of adjacent 2s. This is clearly a necessary condition because if there are an odd number 2m+1 of 1s then the flanking 2s represent the same symbol and so the substring corresponding to 21...12 has |Δ| = 4 + m - (m+1) = 3, which is inadmissible. On the other hand, it is not hard to see that it is also a sufficient condition. We want |Δ| for a substring. In calculating this we can ignore any complete runs of 1s and their flanking 2s, because of the evenness property, so the only possibilities are:

```
some 1s                    |Δ| = 0 or 1;

some 1s, 2                 |Δ| = 1 or 2;

some 1s, 2, some 1s        |Δ| = 1 or 2;

2                          |Δ| = 2;

2, some 1s                 |Δ| = 1 or 2;

```

Now take n even = 2m. The balance requirement implies that each sequence of 1s and 2s can be obtained by starting with 22...2 or 122...21 and replacing zero or more 2s by a pair of 1s. Moreover this process generates each acceptable sequence just once, except for 11...1 which is generated twice. Thus there are 2m + 2m-1 - 1 sequences and hence 2(2m + 2m-1 - 1) = 3·2m - 2 balanced strings. For n odd = 2m+1, we start with 122...2 or 22...21 and get 2(2m + 2m - 1) = 2m+2 - 1 balanced strings.

For a less elegant solution, we try to get a recurrence relation. It quickly becomes clear that we need to distinguish strings which start with a repeated symbol from those which do not. The main task turns out to be showing that the number of balanced strings length n which begin with XX or OO is 2[n/2]. Let b be a balanced string length 2n+1 beginning with XX. Since 2n+1 is odd Δ(b) = 1 or -1. But if Δ = -1, then the substring formed by deleting the initial XX would have Δ = -3. Hence Δ = 1. Similarly, if a balanced string length 2n beginning with XX must have Δ = 0 or 2.

Now we claim that if we add either X or O add the end of a balanced string length 2n+1 beginning with XX to get a new string b, then b is balanced. It is sufficient to show that |Δ(s)| ≤ 2 for a substring ending with the final symbol. Note first that Δ(b) = 0 or 2, depending on whether we added a O or an X. Let t be the complementary string b \ s. Then Δ(s) = Δ(b) - Δ(t) = 0 or 2 less 0, 1, or 2 = something in the range -2 to 2. Thus the number of balanced strings beginning XX and length 2n is twice the number length 2n-1.

On the other hand, if have a balanced string length 2n, the symbol we add must be chosen to fix Δ for the new string at 1, so at most one of X and O will work. In fact a similar argument shows that if we pick X or O to make Δ = 1, then the new string is balanced. Taking b, s and t as before we have Δ(s) = Δ(b) - Δ(t) = 1 less 0, 1 or 2 = 1, 0 or -1. Thus the number of balanced strings beginning XX and length 2n equals the number length 2n+1. A trivial induction now gives that the number length n is 2[n/2]-1. But the numbers of balanced strings beginning XX and OO are obviously the same, so we get the result stated at the start.

Now we claim that the number of balanced strings length n+1 beginning XO is the same as the number of balanced strings length n beginning O. For clearly if b is a balanced string length n+1 beginning XO, then the substring formed by deleting the initial X is balanced, so it is sufficient to show that if we add X at the start of a balanced string b beginning with O, then the new string b' is balanced. So suppose s is a substring of b' beginning with the first symbol X. s has the same Δ as the string formed by deleting its first two symbols, so we must have |Δ| ≤ 2, since b is balanced.

So let un be the number of balanced strings length n. Then we have established that u2n = u2n-1 + 2n, u2n+1 = u2n + 2n. So we get immediately that u2n = (2n + 2n-1) + u2n-2 = ... = (n + 2n-1) + (2n-1 + 2n-2) + ... + (21 + 20) + 1 = 2n + (2n + 2n-1 + ... + 21) = 2n + 2n+1 - 2 = 3·2n - 2 (checking that u2 = 4). Hence u2n+1 = 4·2n - 2 = 2n+2 - 2.