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Asymptote of quasi-geometric series

Source: 2021 Simon Marais, A4

November 2, 2021
calculusreal analysis

Problem Statement

For each positive real number rr, define a0(r)=1a_0(r) = 1 and an+1(r)=ran(r)a_{n+1}(r) = \lfloor ra_n(r) \rfloor for all integers n0n \ge 0. (a) Prove that for each positive real number rr, the limit L(r)=limnan(r)rn L(r) = \lim_{n \to \infty} \frac{a_n(r)}{r^n} exists. (b) Determine all possible values of L(r)L(r) as rr varies over the set of positive real numbers. Here x\lfloor x \rfloor denotes the greatest integer less than or equal to xx.