Let , . Determine the sets that contain and for which is a prime, for all distinct .
Solution
It is easy to see that can not contain more than two numbers of any parity. Combined with , this forces to have exactly elements, two of each parity. The difference between the two even (odd) numbers must be , therefore we can have two types of sets: and .
In the first case, the differences , , and give different remainders upon division by , hence one of these differences has to be . Checking all the possibilities leads to the solutions and .
In the second case, one of the differences , , and has to be . Studying the cases we get the answers , .
In conclusion
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