An invariant refers to a property of a situation that remains the same after multiple given operations.
The positive integers through are written on a blackboard. At any given point, Evan can erase any three numbers , , and and replace them with . What is the greatest number that can appear on the board at any given point?
2011 IMO Problem 2 (it is highly recommended that students watch the video solution, given the difficulty of the IMO)
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