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%I #8 Jun 07 2019 09:08:11
%S 3,4,6,7,12,13,23,28,42,51,66,247,291,623,3429,3892,4416,7366,7991,
%T 8737,10151,10209,40129,41604,50818,68088
%N Numbers k such that 7*10^k - 89 is prime.
%C For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 11 is prime (see Example section).
%C a(27) > 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 69w11</a>.
%e 4 is in this sequence because 7*10^4 - 89 = 69911 is prime.
%e Initial terms and primes associated:
%e a(1) = 3, 6911;
%e a(2) = 4, 69911;
%e a(3) = 6, 6999911;
%e a(4) = 7, 69999911;
%e a(5) = 12, 6999999999911; etc.
%t Select[Range[2, 100000], PrimeQ[7*10^# - 89] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jan 31 2017
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