MathDB
Turkey Junior Olympiad 2009, Part II - P2

Source:

January 20, 2013

Problem Statement

In the beginnig, each square of a strip formed by nn adjacent squares contains 00 or 11. At each step, we are writing 11 to the squares containing 00 and to the squares having exactly one neighbour containing 11, and we are writing 00s into the other squares.
Determine all possible values of nn such that whatever the initial arrangement of 00 and 11 is, after finite number of steps, all squares can turn into 00.