2006 USAMO Problems/Problem 2
For a given positive integer find, in terms of , the minimum value of for which there is a set of distinct positive integers that has sum greater than but every subset of size has sum at most .
Let one optimal set of integers be with .
The two conditions can now be rewritten as and . Subtracting, we get that , and hence . In words, the sum of the smallest numbers must exceed the sum of the largest ones.
Let . As all the numbers are distinct integers, we must have , and also .
Thus we get that , and .
As we want the second sum to be larger, clearly we must have . This simplifies to .
Hence we get that:
On the other hand, for the set the sum of the largest elements is exactly , and the sum of the entire set is , which is more than twice the sum of the largest set.
Hence the smallest possible is .
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