MathDB
JBMO Shortlist 2020 C1

Source: JBMO Shortlist 2020

July 4, 2021
JuniorBalkanshortlist2020combinatoricsgame

Problem Statement

Alice and Bob play the following game: starting with the number 22 written on a blackboard, each player in turn changes the current number nn to a number n+pn + p, where pp is a prime divisor of nn. Alice goes first and the players alternate in turn. The game is lost by the one who is forced to write a number greater than 22...22020\underbrace{22...2}_{2020}. Assuming perfect play, who will win the game.