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%I #11 Jan 17 2019 13:44:10
%S 2,3,6,12,39,45,50,75,126,282,431,459,1557,2724,3453,4058,15879,24209,
%T 31844,61760,155133
%N Numbers k such that (17*10^k - 89)/9 is prime.
%C For k > 1, numbers such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 79 is prime (see Example section).
%C a(22) > 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 18w79</a>
%e 2 is in this sequence because (17*10^2 - 89)/9 = 179 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 179;
%e a(2) = 3, 1879;
%e a(3) = 6, 1888879;
%e a(4) = 12, 1888888888879;
%e a(5) = 39, 1888888888888888888888888888888888888879; etc.
%t Select[Range[1, 100000], PrimeQ[(17*10^# - 89)/9] &]
%o (PARI) isok(k) = isprime((17*10^k - 89)/9); \\ _Michel Marcus_, Nov 20 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 19 2017
%E a(21) from _Robert Price_, Jul 23 2018
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