2022 Polish Junior Math Olympiad Finals P2
Source: 2022 Polish Junior Math Olympiad Finals P2
May 21, 2023
number theory
Problem Statement
Find all positive integers for which both numbers 1\;\;\!\!\!\!\underbrace{77\ldots 7}_{\text{$n$ sevens}}\!\!\!\! \text{and} 3\;\; \!\!\!\!\underbrace{77\ldots 7}_{\text{$n$ sevens}} are prime.