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Combinatorics Difficulty 4.9 AIME Prove it

Two permutations a1,a2,,a2010a_1, a_2, \ldots, a_{2010} and
b1,b2,,b2010b_1, b_2, \ldots, b_{2010} of the numbers 1,2,,20101, 2, \ldots, 2010
are said to intersect if ak=bka_k = b_k for some value of kk in the
range 1k20101 \le k\le 2010. Show that there exist 10061006 permutations
of the numbers 1,2,,20101, 2, \ldots, 2010 such that any other such
permutation is guaranteed to intersect at least one of these 10061006
permutations.

Solution

Let nn be a positive integer. Let mm be the smallest positive integer with
m>nmm > n - m. Since n>nn=0n > n - n = 0, mnm \le n. Let N={1,,n}N = \{1, \ldots, n\}
be the set of positive integers from 11 to nn. Let MNM \subset N,
be M={1,,m}M = \{1, \ldots, m\}.
Let PnP_n be the set of of permutations of NN.
Let CmC_m be the set of cyclic permutations of MM, there are mm
cyclic permutations in all, and CmC_m acts transitively on MM, i.e.
for every pair of elements a,bMa,b \in M, there is an element of CmC_m
that maps aa to bb.
Let CmPnC'_m \subset P_n be the permutations in PnP_n that leave NMN\setminus M
fixed, and restricted to MM yield one of the permutations in CmC_m.
There is a natural one-to-one correspondence between CmC'_m and CmC_m.
We claim that the mm permutations CmC'_m intersect every permutation in
PnP_n.
Suppose, to the contrary, that there exists a permutation pPnp \in P_n
that does not intersect any permutation in CmC'_m. Since CmC'_m acts
transitively on MNM \subset N the permutation pp cannot send any element of
MM to any other element of MM, therefore it must send all the
elements of MM to NMN\setminus M, but since NMN\setminus M has nmn - m elements and m>nmm > n - m, this is impossible
by the pigeonhole principle. Therefore such a permutation cannot
exist, and the permutations in CmC'_m intersect every permutation in
PnP_n.
For n=2010n = 2010 we get m=1006m = 1006, which is the required special
case of the general result above.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.