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Putnam 1980 B4

  • Session B

Let A1,A2,…,A1066A_1 , A_2 ,\ldots, A_{1066} be subsets of a finite set XX such that ∣Ai∣>12∣X∣|A_i | > \frac{1}{2} |X| for 1≤i≤1066.1\leq i \leq 1066. Prove that there exist ten elements x1,x2,…,x10x_1 ,x_2 ,\ldots , x_{10} of XX such that every AiA_i contains at least one of x1,x2,…,x10.x_1 , x_2 ,\ldots, x_{10}.

Dataset text AI-MO AoPS. Quoted with source.

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