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51st Ukrainian National Mathematical Olympiad, 3rd Round

Ukraine counting and probability

Problem

There are piles of stones. The first pile contains stones, the second pile contains stones, the third pile contains stones, and so on. The -th pile contains stones. At one step, one can pick three piles and add stones to the first pile, stones to the second pile, and stones to the third pile, or pick any three piles and take stones out of the first pile, stones out of the second pile, and stones out of the third pile, provided that each pile has enough stones. Is it possible after a finite number of such operations to get exactly stones in each pile?
Solution
After each operation the total number of stones changes by a number which is divisible by . At the end, the total number is , which is divisible by . However, at the starting moment the total number is , which is not divisible by . Indeed, , , therefore
Final answer
No

Techniques

Invariants / monovariantsMultiplicative orderSums and products