MathDB
(-1)^{P(1)},(-1)^{P(2)},(-1)^{P(3)},... (I Soros Olympiad 1994-95 R1 11.8)

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August 1, 2021
polynomialalgebraperiodic

Problem Statement

A polynomial with rational coefficients is called integer, if it takes integer values ​​for all integer values ​​of the variable. For an integer polynomial PP, consider the sequence (1)P(1),(1)P(2),(1)P(3),...(-1)^{P(1)},(-1)^{P(2)},(-1)^{P(3)},...
a) Prove that this sequence is periodic, the period of which is some power of two (i.e. for some integer kk and for all natural ii, the ii-th and (i+2ki+2^k)th members of the sequence are equal).
b) Prove that for any periodic sequence consisting of (1)(- 1) and 1 1 and with a period of some power of two, there exists a integer, polynomial P for which this sequence is (1)P(1),(1)P(2),(1)P(3),...(-1)^{P(1)},(-1)^{P(2)},(-1)^{P(3)},...