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Mongolian National Mathematical Olympiad

Mongolia counting and probability

Problem

girls are standing in a circle, each holding exactly 1 card. One girl gives her card to the girl on her left, who in turn gives 2 cards to the girl on her left. The girl who got the cards gives 1 card to the girl on her left. The girl who got the card gives 2 cards to the girl on her left. Continuing this way, each girl gives alternating 1 or 2 cards to the girl on her left. Anyone who has no card leaves the game immediately. Find all values of such that all cards are collected by one girl.
Solution
Answer: , , , . Solution omitted.
Final answer
n = 2, or n = 2^k + 1, or n = 2^k + 2 for k ≥ 1

Techniques

Invariants / monovariantsGames / greedy algorithms