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romania tst

Source: Romania TST for IMO 1994 second exam

September 6, 2017
combinatoricsalgebra

Problem Statement

Let a1,a2,...,ana_1, a_2, . . ., a_n be a finite sequence of 00 and 11. Under any two consecutive terms of this sequence 00 is written if the digits are equal and 11 is written otherwise. This way a new sequence of length n1n -1 is obtained. By repeating this procedure n1n - 1 times one obtains a triangular table of 00 and 11. Find the maximum possible number of ones that can appear on this table