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IMO 2025 — Complete Shortlist Question 1

IMO Shortlist · IMO 2025 — Complete Shortlist

Question 1

Quadratic solitaire is a single-player game. To start the game, the player chooses two distinct nonzero integers $a$ and $b$ and writes the equation $x^{2}+ax+b=0$ on a blackboard. On each turn, if the equation currently written on the blackboard has two distinct nonzero integer solutions $x=u$ and $x=v$, then the player erases the equation and replaces it by one of the equations $x^{2}+ux+v=0$ or $x^{2}+vx+u=0$ of their choosing. Otherwise, the game ends.

Determine all initial choices of a and b such that quadratic solitaire can be played forever.