MathDB
Sequence with strange inequality relation

Source: 2018 South Korea USCM P8

August 14, 2020
inequalitiesSequenceseriescollege contests

Problem Statement

Suppose a sequence of reals {an}n0\{a_n\}_{n\geq 0} satisfies a0=0a_0 = 0, 100101<a100<1\frac{100}{101} <a_{100}<1, and 2anan1an+12(1an)32a_n - a_{n-1} -a_{n+1} \leq 2 (1-a_n )^3 for every n1n\geq 1. (1) Define a sequence bn=annn+1b_n = a_n - \frac{n}{n+1}. Prove that bnbn+1b_n\leq b_{n+1} for any n100n\geq 100. (2) Determine whether infinite series n=1ann2\sum_{n=1}^\infty \frac{a_n}{n^2} converges or diverges.