Let (an) be a sequence of integers, with a1=1 and for evert integer n≥1, a2n=an+1 and a2n+1=10an. How many times 111 appears on this sequence?
Problem
Source: Rio de Janeiro Mathematical Olympiad 2018, Level 4, #2
Tags: number theory, combinatorics
18.11.2019 02:09
If no ×10 steps, we get 1 routes. If 1 step of ×10 then there are 11 routes since this step happens before you exceed 11. If 2 step of ×10 then there are 2 routes since the first such step must happen immediately. There are no routes involving more than 2 steps of ×10. That is a total of 14 routes. This is also how many times 111 happens since every positive integer is uniquely reachable from 1 by a sequence of n↦2n and n↦2n+1 moves. The indices where 111 happens are: 14 4098 14336 2099200 1075838976 551903297536 283673999966208 146366987889541120 76092819304051900416 40140115104391984316416 21760664753063325144711168 12379400392853802748991242240 7605903601369376408980219232256 1298074214633706907132624082305024
18.11.2019 18:07