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International olympiad of metropolises 2016 P2

Source: International olympiad of metropolises 2016

September 7, 2016
inequalitiesalgebracombinatorics

Problem Statement

Let a1,...,ana_1, . . . , a_n be positive integers satisfying the inequality i=1n1an12\sum_{i=1}^{n}\frac{1}{a_n}\le \frac{1}{2}. Every year, the government of Optimistica publishes its Annual Report with n economic indicators. For each i=1,...,ni = 1, . . . , n,the possible values of the ithi-th indicator are 1,2,...,ai1, 2, . . . , a_i. The Annual Report is said to be optimistic if at least n1n - 1 indicators have higher values than in the previous report. Prove that the government can publish optimistic Annual Reports in an infinitely long sequence.