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Decimal expansion has n - 1 digits 1, one digit 7 is prime

Source: IMO ShortList 1990, Problem 27 (USS 1)

August 15, 2008
number theorydecimal representationprime numbersDigitsIMO Shortlist

Problem Statement

Find all natural numbers n n for which every natural number whose decimal representation has n \minus{} 1 digits 1 1 and one digit 7 7 is prime.
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