27

Part of 1990 IMO Shortlist

Problems(1)

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

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

Find all natural numbers

n

for which every natural number whose decimal representation has n \minus{} 1 digits

1

7

number theorydecimal representationprime numbersDigitsIMO Shortlist