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MathNet 2019-part-01 — 184 problems (EN–VI) Question 77

Math Olympiad Vault · MathNet 2019-part-01 — 184 problems (EN–VI)

Question 77

Let $S$ be a finite set with $n \ge 2$ elements. Two players, $A$ and $B$, alternately choose nonempty proper subsets of $S$, where

(1) it is not allowed to choose a set that contains a set previously chosen by any player,

(2) it is not allowed to choose a set that is contained in any previously chosen set,

(3) it is not allowed to choose a set whose union with any previously chosen set is $S$.

$A$ begins. The player who first cannot choose a set anymore loses. Which player has a winning strategy?