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Number theory Difficulty 6.0 National Olympiad Prove it Romania

We call a composite positive integer nn nice if it is possible to arrange its factors that are larger than 11 on a circle such that two neighboring numbers are not coprime. How many of the elements of the set {1,2,3,,100}\{1, 2, 3, \ldots, 100\} are nice?

Solution

If n=pqn = pq, where p,qp, q are distinct primes, it is clear that we can not arrange p,qp, q and pqpq without pp and qq being neighbors, therefore pqpq is not nice.

If nn is not a product of two distinct primes, then n=p1α1p2α2pkαkn = p_1^{\alpha_1} p_2^{\alpha_2} \dots p_k^{\alpha_k}, where k1k \ge 1 and αi1\alpha_i \ge 1. A convenient way of arranging the factors of nn, larger than 11, on a circle is the following: we write the numbers in a succession of the form:

n,S1,S2,,Sk1,Skn, S_1, S_2, \dots, S_{k-1}, S_k

where:

* S1S_1 is a sequence of numbers that contains all the factors of nn, other than nn, that are multiples of p1p_1, the last one in the sequence being p1p2p_1p_2;
* S2S_2 is a sequence of numbers that contains all the factors of nn that are multiples of p2p_2, but not of p1p_1, the last number in the sequence being p2p3p_2p_3;
* S3S_3 is a sequence of numbers that contains all the factors of nn that are multiples of p3p_3, but are multiples of neither p1p_1 nor p2p_2, the last number in the sequence being p3p4p_3p_4;
... and so on.

The set {1,2,,100}\{1, 2, \ldots, 100\} contains 7474 composite numbers, 3030 of which being of the form pqpq, with p,qp, q distinct primes. This leaves 4444 nice numbers.

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