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2.

Part of 2022 Polish Junior Math Olympiad Finals

Problems(1)

2022 Polish Junior Math Olympiad Finals P2

Source: 2022 Polish Junior Math Olympiad Finals P2

5/21/2023
Find all positive integers nn for which both numbers 1\;\;\!\!\!\!\underbrace{77\ldots 7}_{\text{$n$ sevens}}\!\!\!\! \text{and}  3\;\; \!\!\!\!\underbrace{77\ldots 7}_{\text{$n$ sevens}} are prime.
number theory