Let $p$ be a prime number such that $p = 10^{d -1} + 10^{d-2} + ...+ 10 + 1$. Show that $d$ is a prime.
Problem
Source: OLCOMA Costa Rica National Olympiad, Final Round, 2018 Shortlist N4
Tags: prime, number theory
Source: OLCOMA Costa Rica National Olympiad, Final Round, 2018 Shortlist N4
Tags: prime, number theory
Let $p$ be a prime number such that $p = 10^{d -1} + 10^{d-2} + ...+ 10 + 1$. Show that $d$ is a prime.