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%I #6 Aug 16 2019 22:29:18
%S 1,3,6,35,51,84,123,1589,1591,4911,6221,48645,78504,107355
%N Numbers k such that 5*10^k + 11 is prime.
%C For k>1, numbers such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 11 is prime (see Example section).
%C a(15) > 2*10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/primedifficulty.txt">Search for 50w11</a>.
%e 6 is in this sequence because 5*10^4 + 11 = 5000011 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 61;
%e a(2) = 3, 5011;
%e a(3) = 6, 5000011;
%e a(4) = 35, 500000000000000000000000000000000011; etc.
%t Select[Range[0, 200000], PrimeQ[5*10^# + 11] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Aug 16 2019
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