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d is prime if prime p = 10^{d -1} + 10^{d-2} + ...+ 10 + 1

Source: OLCOMA Costa Rica National Olympiad, Final Round, 2018 Shortlist N4

October 1, 2021
primenumber theory

Problem Statement

Let pp be a prime number such that p=10d−1+10d−2+...+10+1p = 10^{d -1} + 10^{d-2} + ...+ 10 + 1. Show that dd is a prime.
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