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Number theory Difficulty 6.0 National Olympiad Prove it Romania

Let nNn \in \mathbb{N}, n4n \ge 4. Determine the sets A={a1,a2,,an}NA = \{a_1, a_2, \dots, a_n\} \subset \mathbb{N} that contain 20152015 and for which aiaj|a_i - a_j| is a prime, for all distinct i,j{1,2,,n}i, j \in \{1, 2, \dots, n\}.

Solution

It is easy to see that AA can not contain more than two numbers of any parity. Combined with n4n \ge 4, this forces AA to have exactly 44 elements, two of each parity. The difference between the two even (odd) numbers must be 22, therefore we can have two types of sets: {2013,2015,2k,2k+2}\{2013, 2015, 2k, 2k + 2\} and {2015,2017,2k,2k+2}\{2015, 2017, 2k, 2k + 2\}.

In the first case, the differences (2k+2)2013|(2k + 2) - 2013|, 2k2013|2k - 2013|, and 2k2015|2k - 2015| give different remainders upon division by 33, hence one of these differences has to be 33. Checking all the possibilities leads to the solutions A={2013,2015,2008,2010}A = \{2013, 2015, 2008, 2010\} and A={2013,2015,2018,2020}A = \{2013, 2015, 2018, 2020\}.

In the second case, one of the differences (2k+2)2015|(2k + 2) - 2015|, 2k2015|2k - 2015|, and 2k2017|2k - 2017| has to be 33. Studying the cases we get the answers {2015,2017,2010,2012}\{2015, 2017, 2010, 2012\}, {2015,2017,2020,2022}\{2015, 2017, 2020, 2022\}.

In conclusion
A1={2008,2010,2013,2015},A2={2010,2012,2015,2017},A3={2013,2015,2018,2020},A4={2015,2017,2020,2022}. \begin{aligned} A_1 &= \{2008, 2010, 2013, 2015\}, & A_2 &= \{2010, 2012, 2015, 2017\}, \\ A_3 &= \{2013, 2015, 2018, 2020\}, & A_4 &= \{2015, 2017, 2020, 2022\}. \end{aligned}

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