Maths Olympiad Prep

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Combinatorics Difficulty 5.2 AIME, harder Prove it United States

Problem:
Let π\pi be a permutation of the numbers from 11 through 20122012. What is the maximum possible number of integers nn with 1n20111 \leq n \leq 2011 such that π(n)\pi(n) divides π(n+1)\pi(n+1)?

Solution

Solution:
Answer: 10061006

Since any proper divisor of nn must be less than or equal to n/2n / 2, none of the numbers greater than 10061006 can divide any other number less than or equal to 20122012. Since there are at most 10061006 values of nn for which π(n)1006\pi(n) \leq 1006, this means that there can be at most 10061006 values of nn for which π(n)\pi(n) divides π(n+1)\pi(n+1).

On the other hand, there exists a permutation for which π(n)\pi(n) divides π(n+1)\pi(n+1) for exactly 10061006 values of nn, namely the permutation:
(1,2,22,23,,210,3,23,223,233,,293,5,) \left(1,2,2^{2}, 2^{3}, \ldots, 2^{10}, 3,2 \cdot 3,2^{2} \cdot 3,2^{3} \cdot 3, \ldots, 2^{9} \cdot 3,5, \ldots\right)
Formally, for each odd number 2012\ell \leq 2012, we construct the sequence ,2,4,,2k\ell, 2 \ell, 4 \ell, \ldots, 2^{k} \ell, where kk is the largest integer such that 2k20122^{k} \ell \leq 2012. We then concatenate all of these sequences to form a permutation of the numbers 11 through 20122012 (note that no number occurs in more than one sequence). It follows that if π(n)1006\pi(n) \leq 1006, then π(n+1)\pi(n+1) will equal 2π(n)2 \pi(n), and therefore π(n)\pi(n) will divide π(n+1)\pi(n+1) for all 10061006 values of nn satisfying 1π(n)10061 \leq \pi(n) \leq 1006.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.