Problem

Source: 49th Austrian Mathematical Olympiad National Competition (Final Round) 28th April 2018 p3

Tags: combinatorics, game, number theory, decimal representation, Digits



Alice and Bob determine a number with $2018$ digits in the decimal system by choosing digits from left to right. Alice starts and then they each choose a digit in turn. They have to observe the rule that each digit must differ from the previously chosen digit modulo $3$. Since Bob will make the last move, he bets that he can make sure that the final number is divisible by $3$. Can Alice avoid that? (Proposed by Richard Henner)