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Algebra Difficulty 6.3 National Olympiad Prove it Iran

Let (an)(a_n) be a sequence of positive real numbers such that for all n>2025n > 2025 we have
an=max1i2025animin1i2025ani a_n = \max_{1 \le i \le 2025} a_{n-i} - \min_{1 \le i \le 2025} a_{n-i}

Prove that there is a positive integer MM such that an<11404a_n < \frac{1}{1404}, for all n>Mn > M.

Solution

First, we prove that the sequence is bounded; divide the sequence into blocks of 20252025 terms. It can be easily seen that the maximum of these blocks is decreasing. If the infimum of the maximums of the blocks is zero, the statement follows. Therefore, assume this infimum is MM. Due to the decreasing nature of the maximums, it follows that the infimum of the maximums of all consecutive blocks of 20252025 terms (not just the initial blocks) is equal to MM. In fact, beyond a certain point, the maximum of each 20252025-term block will be between MM and M+ϵM + \epsilon.
This implies that the minimum in each of these blocks is smaller than ϵ\epsilon. Consider a term in the sequence from this point onwards that is smaller than ϵ\epsilon. According to the inequality above, each of the next 20252025 terms will be in the interval [Mϵ,M+ϵ][M - \epsilon, M + \epsilon].
Suppose the largest of these terms is M+ϵM + \epsilon and the smallest of these terms is M+ϵM + \epsilon'. If ϵ<0\epsilon' < 0, then the 20262026th term will be greater than ϵ\epsilon, which implies that the supremum of the next 20252025 terms becomes less than MM, which is a contradiction.
Therefore, from a certain point onwards, the sequence will consist of 20252025 terms larger than MM and one small term. More precisely, the sequence will be of the form
M+ϵ1,M+ϵ2,,M+ϵ2025,ϵ=max(ϵi)min(ϵi) M + \epsilon_1, M + \epsilon_2, \dots, M + \epsilon_{2025}, \epsilon = \max(\epsilon_i) - \min(\epsilon_i)
In the next block, the first term will be larger than all terms. Therefore, we can assume ϵ1>ϵi\epsilon_1 > \epsilon_i. This easily implies that if the next block is
M+ϵ1,M+ϵ2,,M+ϵ2025,ϵ=max(ϵi)min(ϵi) M + \epsilon'_1, M + \epsilon'_2, \dots, M + \epsilon'_{2025}, \epsilon' = \max(\epsilon'_i) - \min(\epsilon'_i)
we have
ϵ1=ϵ1ϵ,ϵi<ϵ12ϵ    ϵϵ \epsilon'_1 = \epsilon_1 - \epsilon, \epsilon'_i < \epsilon_1 - 2\epsilon \implies \epsilon' \ge \epsilon

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