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Number theory Difficulty 7.5 National olympiad, round 2 Find the answer

Find, with proof, the least integer NN such that if any 20162016 elements are removed from the set {1,2,...,N}\{1, 2,...,N\} , one can still find 20162016 distinct numbers among the remaining elements with sum NN .

A number or a short expression. Spacing and $ signs are ignored.

Solution

Since any 20162016 elements are removed, suppose we remove the integers from 11 to 20162016 . Then the smallest possible sum of 20162016 of the remaining elements is 2017+2018++4032=10086049=60973922017+2018+\cdots + 4032 = 1008 \cdot 6049 = 6097392 so clearly N6097392N\ge 6097392 . We will show that N=6097392N=6097392 works.
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{1,26097392}\{1,2\cdots 6097392\} contain the integers from 11 to 60486048 , so pair these numbers as follows:
1,60481, 6048
2,60472, 6047
3,60463, 6046
\cdots
3024,30253024, 3025
When we remove any 20162016 integers from the set {1,2,N}\{1,2,\cdots N\} , clearly we can remove numbers from at most 20162016 of the 30243024 pairs above, leaving at least 10081008 complete pairs. To get a sum of NN , simply take these 10081008 pairs, all of which sum to 60496049 . The sum of these 20162016 elements is 10086049=60973921008 \cdot 6049 = 6097392 , as desired.
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We have shown that NN must be at least 60973926097392 , and that this value is attainable. Therefore our answer is 6097392\boxed{6097392} .
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