%I #12 Jun 09 2024 18:05:28
%S 2,3,8,12,24,105,140,231,234,260,294,354,602,813,1562,2961,3450,5622,
%T 8121,9014,23052,38505,41139,63576
%N Numbers k such that (107*10^k - 17)/9 is prime.
%C For k > 0, numbers k such that the digits 11 followed by k-1 occurrences of the digit 8 followed by the digit 7 is prime (see Example section).
%C a(25) > 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/prime_difficulty.txt">Search for 118w7</a>.
%e 3 is in this sequence because (107*10^3 - 17)/9 = 11887 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 1187;
%e a(2) = 3, 11887;
%e a(3) = 8, 1188888887;
%e a(4) = 12, 11888888888887;
%e a(5) = 24, 11888888888888888888888887; etc.
%t Select[Range[0, 100000], PrimeQ[(107*10^# - 17)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Feb 10 2017