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Combinatorics Difficulty 7.1 National olympiad, round 2 Prove it Bulgaria

One cuts a paper strip of length 20072007 into two parts of integer lengths and writes down the two integers on the board. Then cuts one of the two parts into two parts of integer lengths and writes down the two integers on the board. The cutting stops when all parts are of length 11. A cut is called bad if the two parts obtained are not of equal lengths.

a) Find the minimum possible bad cuts.

b) Prove that for all cuttings with minimum possible bad cuts the number of distinct integers on the board is one and the same.

Solution

a) Let the length of the strip be nn. Denote by g(n)g(n) and f(n)f(n) respectively the number of 11's in the binary representation of nn and the minimum possible number of bad cuts. Let n=2k1+2k2++2kln = 2^{k_1} + 2^{k_2} + \cdots + 2^{k_l}. Consider the following sequence of cuts: first cut a strip of length 2k12^{k_1}, next cut a strip of length 2k22^{k_2} and so on. After the last cut we obtain two strips of lengths 2kl12^{k_{l-1}} and 2kl2^{k_l}. Note that a strip whose length is a power of 22 can be cut into strips of length 11 without bad cuts. Therefore the number of bad cuts equals l1l-1, i.e.
(1)f(n)g(n)1. (1) \qquad f(n) \le g(n) - 1.
We prove by induction on nn that f(n)g(n)1f(n) \ge g(n) - 1. For n=1n=1 we have f(1)=0f(1) = 0 and g(1)=1g(1) = 1, i.e. the statement is true. Suppose it is true for all nkn \le k, where kk is a positive integer and let n=k+1n = k+1.

1. Suppose the first cut is bad and it leaves two strips of lengths aa and bb. Then a+b=k+1a+b = k+1 and f(k+1)=1+f(a)+f(b)f(k+1) = 1 + f(a) + f(b). If the binary representations of aa and bb have no common digit 11 then g(k+1)=g(a)+g(b)g(k+1) = g(a) + g(b) and therefore
f(k+1)=1+f(a)+f(b)1+g(a)1+g(b)1=g(k+1)1. f(k+1) = 1 + f(a) + f(b) \ge 1 + g(a) - 1 + g(b) - 1 = g(k+1) - 1.
If the binary representations of aa and bb have at least one common digit 11 then g(k+1)g(a)+g(b)1g(k+1) \le g(a) + g(b) - 1 and therefore
f(k+1)=1+f(a)+f(b)1+g(a)1+g(b)1g(k+1)>g(k+1)1. f(k+1) = 1 + f(a) + f(b) \ge 1 + g(a) - 1 + g(b) - 1 \ge g(k+1) > g(k+1) - 1.

2. Suppose the first cut is not bad, i.e. the strip is cut into two parts each of length aa. Then k+1=2ak+1 = 2a and g(k+1)=g(a)g(k+1) = g(a). If g(k+1)=1g(k+1) = 1 then f(k+1)=0f(k+1) = 0 and the statement is true. Otherwise
f(k+1)=f(a)+f(b)=2f(a)2g(a)2=2g(k+1)2>g(k+1)1. f(k+1) = f(a) + f(b) = 2f(a) \ge 2g(a) - 2 = 2g(k+1) - 2 > g(k+1) - 1.
Thus, when g(k+1)>1g(k+1) > 1 we have f(k+1)>g(k+1)1f(k+1) > g(k+1) - 1.

This proves the induction hypothesis giving
(2)f(n)g(n)1. (2) \qquad f(n) \geq g(n) - 1.
It follows from (1) and (2) that f(n)=g(n)1f(n) = g(n) - 1.

a) Since the binary representation of 20072007 is 11110101111111010111, i.e. g(2007)=9g(2007) = 9, we obtain f(2007)=8f(2007) = 8.

b) It follows from the above arguments that if the number of bad cuts is f(n)=g(n)1f(n) = g(n) - 1 then every bad cut leaves two parts of lengths aa and bb such that the binary representations of aa and bb have no common digit 11. Moreover the good cuts are done only over strips whose lengths are powers of 22. It is clear that rearranging the cuts we may assume that the first cuts are bad. Their number equals g(n)1g(n) - 1 and each bad cut gives two new numbers on the table. Therefore after all bad cuts we have 2g(n)22g(n) - 2 distinct numbers. The powers of 22 that appear are all powers up to the highest power in the binary representation of nn.
Thus, the number of distinct numbers on the table equals 2g(n)2+k+1=2g(n)+k12g(n) - 2 + k + 1 = 2g(n) + k - 1, where kk is the highest power of 22 in the binary representation of nn.

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