MathDB
Constant Sequence with Inequality

Source: KMO 2022 P7

October 29, 2022
inequalitiesalgebraSequence

Problem Statement

Suppose that the sequence {an}\{a_n\} of positive reals satisfies the following conditions:
[*]aiaja_i \leq a_j for every positive integers i<ji <j. [*]For any positive integer k3k \geq 3, the following inequality holds: (a1+a2)(a2+a3)(ak1+ak)(ak+a1)(2k+2022)a1a2ak(a_1+a_2)(a_2+a_3)\cdots(a_{k-1}+a_k)(a_k+a_1)\leq (2^k+2022)a_1a_2\cdots a_k
Prove that {an}\{a_n\} is constant.