Can divide the set of integers into 3 subsets such that for any integer , , , and belong to 3 different subsets?
(50th Moscow Mathematical Olympiad, 1987)
Problem 1038
Official solution
[Solution] We prove that it is impossible to divide the set of integers into 3 subsets such that belong to 3 different subsets, where is any integer.
If a 3-tuple of 3 numbers each comes from 3 different subsets, then we call this 3-tuple "representative."
Assume there exists a partition that meets the requirements of the problem, then for any integer , the following 3-tuples are "representative":
We use the notation to indicate that the integer and the integer belong to the same subset, and the notation to indicate that the integer and the integer do not belong to the same subset.
From (2) we know
From (3) we know
From (1) we know
Thus, it can only be that
From (2), by replacing with , we get another "representative" 3-tuple
We can also get (by replacing with )
which is also a "representative" tuple.
Thus, from (5) and (6) we know
Thus, it can only be that
From (4) and (7) we get
Thus , i.e., when , and belong to the same subset, leading to a contradiction. Therefore, there cannot be a partition that meets the requirements of the problem.