IMO Shortlist 1990 problem 27
Dodao/la:
arhiva2. travnja 2012. Find all natural numbers
for which every natural number whose decimal representation has
digits
and one digit
is prime.
%V0
Find all natural numbers $n$ for which every natural number whose decimal representation has $n - 1$ digits $1$ and one digit $7$ is prime.
Izvor: Međunarodna matematička olimpijada, shortlist 1990