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Belarus number theory
Problem
Let be a prime number. Alice and Bob play the following game: they, in turn, select an index in the set that was not selected before by either of the two players and then chooses a digit . Alice starts. The game ends after all the indices have been selected. The goal of Alice is to make the number divisible by , and the goal of Bob is to prevent this. Prove that Alice has the winning strategy.
Solution
2. See IMO-2017 Shortlist, Problem N2.
Techniques
Inverses mod nGames / greedy algorithms