olymmath

ISL 2013 C8

    Players AA and BB play a paintful game on the real line. Player AA has a pot of paint with four units of black ink. A quantity pp of this ink suffices to blacken a (closed) real interval of length pp. In every round, player AA picks some positive integer mm and provides 1/2m1 / 2^{m} units of ink from the pot. Player BB then picks an integer kk and blackens the interval from k/2mk / 2^{m} to (k+1)/2m(k+1) / 2^{m} (some parts of this interval may have been blackened before). The goal of player AA is to reach a situation where the pot is empty and the interval [0,1][0,1] is not completely blackened. Decide whether there exists a strategy for player AA to win in a finite number of moves. (Austria)

    Official text Transcription from NuminaMath olympiads, checked word for word against the official paper. Quoted with source.

    Official text: IMO 2013 Shortlist booklet, page 5.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

    The terms of the source let olymmath link to this solution but not copy it. Read the solution (IMO official site).

    Locator isl/2013/c8 · Tag FJS02YFCVT · Edit this page on GitHub · Keys: j next, k previous