olymmath

ISL 2014 C6

  • Answer

We are given an infinite deck of cards, each with a real number on it. For every real number xx, there is exactly one card in the deck that has xx written on it. Now two players draw disjoint sets AA and BB of 100 cards each from this deck. We would like to define a rule that declares one of them a winner. This rule should satisfy the following conditions: 1. The winner only depends on the relative order of the 200 cards: if the cards are laid down in increasing order face down and we are told which card belongs to which player, but not what numbers are written on them, we can still decide the winner. 2. If we write the elements of both sets in increasing order as A={a1,a2,…,a100}A=\left\{a_{1}, a_{2}, \ldots, a_{100}\right\} and B={b1,b2,…,b100}B=\left\{b_{1}, b_{2}, \ldots, b_{100}\right\}, and ai>bia_{i}>b_{i} for all ii, then AA beats BB. 3. If three players draw three disjoint sets A,B,CA, B, C from the deck, AA beats BB and BB beats CC, then AA also beats CC. How many ways are there to define such a rule? Here, we consider two rules as different if there exist two sets AA and BB such that AA beats BB according to one rule, but BB beats AA according to the other. (Russia)

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

Official text: IMO 2014 Shortlist booklet, page 8.

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Solutions

Official solution Solution 1 by the IMO Problem Selection Committee

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