Maths Olympiad Prep

Library / /26 of 397

Number theory Difficulty 4.7 AIME Prove it Taiwan

Find all composite numbers nn such that the positive divisors of nn greater than 11 can be arranged on a circle so that no two adjacent ones are relatively prime.

Solution

When n=pα,α2n = p^{\alpha}, \alpha \ge 2, arranging them as p,p2,,pαp, p^2, \dots, p^{\alpha} obviously satisfies the condition.

When n=p1α1p2α2pkαk,k3n = p_1^{\alpha_1} p_2^{\alpha_2} \cdots p_k^{\alpha_k}, k \ge 3, first arrange p1p2p3pkp1p_1 p_2 p_3 \cdots p_k p_1 in order on the circle, then arrange the multiples of pjp_j between pj1pjp_{j-1} p_j and pjpj+1p_j p_{j+1}. The resulting arrangement satisfies the condition.

Finally, when n=paqb,a>1n = p^a q^b, a > 1, first place pqpq and p2qp^2 q on the circle, put the multiples of pp on one arc, and the multiples of qq on the other arc.

The remaining case n=pqn = pq cannot be achieved.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.