22. (PHI 1) Prove that the set can be expressed as the disjoint union of 17 subsets such that: (i) each contains the same number of elements; (ii) the sum of all elements of each is the same for .
Solution
22. The statement remains valid if 17 is replaced by any divisor of , , so let be one such divisor. The set can be partitioned as , where . The required statement will be an obvious consequence of the following two claims.
Claim 1. The set can be partitioned into disjoint subsets, each having 3 elements and the same sum.
Proof. Since is odd, let and . For , define
It is easily seen that these three subsets are disjoint and that the sum of elements in each set is .
Claim 2. Each can be partitioned into disjoint subsets, each having 2 elements and the same sum.
Proof. The obvious partitioning works:
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