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Sequences and parity

Source: St. Petersburg MO 2000, 9th grade, P7

April 22, 2023
algebraSequenceParity

Problem Statement

Define a complexity of a set a1,a2,,a_1,a_2,\dots, consisting of 0 and 1 to be the smallest positive integer kk such that for some positive integers ϵ1,ϵ2,,ϵk\epsilon_1,\epsilon_2,\dots, \epsilon_k each number of the sequence ana_n, n>kn>k, has the same parity as ϵ1an1+ϵ2an2++ϵkank\epsilon_1 a_{n-1}+\epsilon_2 a_{n-2}+\dots+\epsilon_k a_{n-k}. Sequence a1,a2,,a_1,a_2,\dots, has a complexity of 10001000. What is the complexity of sequence 1a1,1a2,,1-a_1,1-a_2,\dots,.
[I]Proposed by A. Kirichenko