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Estonian Math Competitions

Estonia number theory

Problem

We call a prime number cute if there exists a prime so that both and are also primes. We call wonderful if both and are cute primes. Find all wonderful numbers.
Solution
Let be a cute prime. The numbers , and give all the possible remainders modulo , so one of them must be divisible by . If , then and , which is impossible, as , are primes. Similarly if , then and , which is also impossible. So , which means that either or . Given that in the case we can have , we have shown that is cute iff both and are primes.

Therefore is wonderful if , , , , and are all primes. We know that the numbers , , , and give all of the possible remainders modulo , so one of those numbers is divisible by . If it's , or , then due to primality it must be equal to , but then is not prime. If it's or , then either or respectively. Thus or . We can verify that both work.
Final answer
3, 5

Techniques

Prime numbersTechniques: modulo, size analysis, order analysis, inequalities