olymmath

USAJMO 2010 Problem 5

  • Day 2

Two permutations a1,a2,…,a2010a_1,a_2,\dots,a_{2010} and b1,b2,…,b2010b_1,b_2,\dots,b_{2010} of the numbers 1,2,…,20101,2,\dots,2010 are said to intersect if ak=bka_k=b_k for some value of kk in the range 1≤k≤20101\le k\le 2010 . Show that there exist 10061006 permutations of the numbers 1,2,…,20101,2,\dots,2010 such that any other such permutation is guaranteed to intersect at least one of these 10061006 permutations.

Dataset text AI-MO AoPS. Quoted with source.

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