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%I #4 May 22 2017 11:58:14
%S 1,253,35473
%N Numbers n such that (10^n-1)/3 * 10^ceiling(log_10(n+1)) + n is prime.
%C No term is a multiple of 2, 3, or 5. The decimal expansion of each corresponding prime is n 3's with n's decimal expansion concatenated. Probable primes found by PrimeForm. Prime for 253 proved by Primo. No more terms up to 50000.
%e The first term is 1 because 31 is prime.
%o (PARI) is(n)=ispseudoprime((10^n-1)/3 * 10^logint(10*n,10) + n) \\ _Charles R Greathouse IV_, May 22 2017
%Y Cf. A070746 (n 1's followed by n is prime), A084428 (n 7's followed by n is prime), A174710, A004218.
%K bref,more,nonn
%O 1,2
%A _Rick L. Shepherd_, Apr 08 2010