Maths Olympiad Prep

Track / Stage 5 / 13 of 400 #613 of 1964

Problem 613

AIME late
Number theory Difficulty 5.0 Prove it Ukrainian National Mathematical Olympiad · Ukraine

Find all pairs of prime numbers (p,q)(p, q) with p>qp > q, for which both numbers p+qp+q and pqp-q are also prime.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

For the number p+qp+q to be prime the numbers pp and qq must be of different parity, which automatically means that q=2q=2, since p>qp > q. By the problem statement we then have that the numbers p2,pp-2, p, and p+2p+2 should be prime. Since they obviously have different remainders in division by 33, one of them must be equal to 33. So we have three possibilities:
* If p2=3p-2=3, then p=5p=5, p+2=7p+2=7, and the pair (5;2)(5; 2) satisfies the problem statement;

* if p=3p=3, then p2=1p-2=1, and is not prime;
* if p+2=3p+2=3, then p=1p=1, and is not prime.
So after considering all possible cases we end up with a single solution p=5,q=2p=5, q=2.

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