MathDB
GMO 2016 #1

Source: GMO 2016

September 7, 2017
GMO-Gulf Mathmatical Olympiadalgebra

Problem Statement

Consider sequences a0a_0,a1a_1,a2a_2,\cdots of non-negative integers defined by selecting any a0a_0,a1a_1,a2a_2 (not all 0) and for each nn \geq 3 letting ana_n = |an1a_n-1 - an3a_n-3|
1-In the particular case that a0a_0 = 1,a1a_1 = 3 and a2a_2 = 2, calculate the beginning of the sequence, listing a0a_0,a1a_1,\cdots,a19a_{19},a20a_{20}.
2-Prove that for each sequence, there is a constant cc such that aia_i \leq cc for all ii \geq 0. Note that the constant cc my depend on the numbers a0a_0,a1a_1 and a2a_2
3-Prove that, for each choice of a0a_0,a1a_1 and a2a_2, the resulting sequence is eventually periodic.
4-Prove that, the minimum length p of the period described in (3) is the same for all permitted starting values a0a_0,a1a_1,a2a_2 of the sequence