Maths Olympiad Prep

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Number theory Difficulty 4.9 AIME Prove it Estonia

Do there exist five distinct prime numbers for which the sum of any three of them is a prime number as well?

Solution

Answer: No.

Assume that there exist five such prime numbers. If there are three among them pairwise incongruent modulo 33, then by adding each of the three separately to the sum of the other two we get three distinct sums modulo 33. One of those three is 00 modulo 33 and therefore is not a prime number. However, if among the five we only get two distinct remainders when dividing by 33, then by the pigeonhole principle there must be three among them that are congruent modulo 33. The sum of those three, however, is divisible by 33 and therefore not a prime number.

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