Olympiad Maths Prep

Track / Stage 6 / 315 of 400 #1315 of 2000

Problem 1315

National olympiad, first round
Number theory Difficulty 6.6 Find the answer

Let nN,n4.n\in \Bbb{N}, n \geq 4. Determine all sets A={a1,a2,...,an}N A = \{a_1, a_2, . . . , a_n\} \subset \Bbb{N} containing 20152015 and having the property that aiaj |a_i - a_j| is prime, for all distinct i,j{1,2,...,n}.i, j\in \{1, 2, . . . , n\}.

Official solution

1. Parity Consideration:
- We start by noting that if three numbers in the set A A have the same parity (all odd or all even), then their pairwise differences would be even. Since the only even prime number is 2, all these differences would have to be 2, which is impossible for three distinct numbers. Therefore, the set A A can contain at most two odd and two even numbers.

2. Set Size:
- Given that n4 n \geq 4 and the set A A must contain exactly four elements (two odd and two even), we can proceed to construct such sets.

3. Odd Elements:
- Since 2015 2015 is an odd number and must be included in the set, the other odd number must be 2 units away from 2015 to ensure the difference is prime. Thus, the other odd number can be either 2013 2013 or 2017 2017 .

4. Even Elements:
- The two even numbers must also be 2 units apart to ensure their difference is prime. These even numbers can either be both less than 2013 or both greater than 2015.

5. Case Analysis:
- Case 1: The odd numbers are 2013 2013 and 2015 2015 .
- The even numbers must be 2 units apart and less than 2013. The only pair that fits this criterion is 2008 2008 and 2010 2010 .
- Thus, one possible set is {2008,2010,2013,2015} \{2008, 2010, 2013, 2015\} .

- Case 2: The odd numbers are 2013 2013 and 2015 2015 .
- The even numbers must be 2 units apart and greater than 2015. The only pair that fits this criterion is 2018 2018 and 2020 2020 .
- Thus, another possible set is {2013,2015,2018,2020} \{2013, 2015, 2018, 2020\} .

- Case 3: The odd numbers are 2015 2015 and 2017 2017 .
- The even numbers must be 2 units apart and less than 2015. The only pair that fits this criterion is 2010 2010 and 2012 2012 .
- Thus, another possible set is {2010,2012,2015,2017} \{2010, 2012, 2015, 2017\} .

- Case 4: The odd numbers are 2015 2015 and 2017 2017 .
- The even numbers must be 2 units apart and greater than 2017. The only pair that fits this criterion is 2020 2020 and 2022 2022 .
- Thus, another possible set is {2015,2017,2020,2022} \{2015, 2017, 2020, 2022\} .

6. Verification:
- We verify that in each of these sets, the pairwise differences are prime:
- For {2008,2010,2013,2015} \{2008, 2010, 2013, 2015\} :
- Differences: 2,5,7,3,5,2 2, 5, 7, 3, 5, 2 (all prime).
- For {2013,2015,2018,2020} \{2013, 2015, 2018, 2020\} :
- Differences: 2,5,7,3,5,2 2, 5, 7, 3, 5, 2 (all prime).
- For {2010,2012,2015,2017} \{2010, 2012, 2015, 2017\} :
- Differences: 2,5,7,3,5,2 2, 5, 7, 3, 5, 2 (all prime).
- For {2015,2017,2020,2022} \{2015, 2017, 2020, 2022\} :
- Differences: 2,5,7,3,5,2 2, 5, 7, 3, 5, 2 (all prime).

The final answer is {2008,2010,2013,2015},{2013,2015,2018,2020},{2010,2012,2015,2017},{2015,2017,2020,2022} \boxed{ \{2008, 2010, 2013, 2015\}, \{2013, 2015, 2018, 2020\}, \{2010, 2012, 2015, 2017\}, \{2015, 2017, 2020, 2022\} } .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.