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Possible values of t/s for stones in buckets

Source: APMO 2020 Problem 5

June 9, 2020
algebranumber theoryAPMOalgorithm

Problem Statement

Let n3n \geq 3 be a fixed integer. The number 11 is written nn times on a blackboard. Below the blackboard, there are two buckets that are initially empty. A move consists of erasing two of the numbers aa and bb, replacing them with the numbers 11 and a+ba+b, then adding one stone to the first bucket and gcd(a,b)\gcd(a, b) stones to the second bucket. After some finite number of moves, there are ss stones in the first bucket and tt stones in the second bucket, where ss and tt are positive integers. Find all possible values of the ratio ts\frac{t}{s}.