MathDB
2015-2016 Spring OMO #19

Source:

March 29, 2016
Online Math Open

Problem Statement

Let Z0\mathbb{Z}_{\ge 0} denote the set of nonnegative integers.
Define a function f:Z0Zf:\mathbb{Z}_{\ge 0} \to\mathbb{Z} with f(0)=1f\left(0\right)=1 and f(n)=512n/10f(n/10) f\left(n\right)=512^{\left\lfloor n/10 \right\rfloor}f\left(\left\lfloor n/10 \right\rfloor\right) for all n1n \ge 1. Determine the number of nonnegative integers nn such that the hexadecimal (base 1616) representation of f(n)f\left(n\right) contains no more than 25002500 digits.
Proposed by Tristan Shin