MathDB
Weird binary sequence

Source: 239 MO 2024 J8

May 22, 2024
combinatorics

Problem Statement

Let x1,x2,x_1, x_2, \ldots be a sequence of 0,10,1, such that it satisfies the following three conditions: 1) x2=x100=1x_2=x_{100}=1, xi=0x_i=0 for 1i1001 \leq i \leq 100 and i2,100i \neq 2,100; 2) x2n1=xn50+1,x2n=xn50x_{2n-1}=x_{n-50}+1, x_{2n}=x_{n-50} for 51n10051 \leq n \leq 100; 3) x2n=xn50,x2n1=xn50+xn100x_{2n}=x_{n-50}, x_{2n-1}=x_{n-50}+x_{n-100} for n>100n>100. Show that the sequence is periodic.