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infinite periodic sequence of 2 letters - All-Russian MO 1996 Regional (R4) 11.8

Source:

September 23, 2024
combinatoricsperiodic

Problem Statement

Is there an infinite periodic sequence consisting of the letters aa andb b, such that if all letters are replaced simultaneously aa to abaaba and letters bb to bbabba does it transform into itself (possibly with a shift)? (A sequence is called periodic if there is such natural number nn, which for every i=1,2,...ii = 1, 2, . . . i-th member of this sequence is equal to the (i+ni + n)- th.)