Problem

Source: 2006 MOP Homework Blue Combinatorics 4

Tags: combinatorics, game, winning strategy, game strategy



For positive integers $t,a$, and $b$, Lucy and Windy play the $(t,a,b)$- game defined by the following rules. Initially, the number $t$ is written on a blackboard. On her turn, a player erases the number on the board and writes either the number $t - a$ or $t - b$ on the board. Lucy goes first and then the players alternate. The player who first reaches a negative losses the game. Prove that there exist infinitely many values of $t$ in which Lucy has a winning strategy for all pairs $(a, b)$ with $a + b = 2005$.