The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #12 Jun 09 2024 18:05:52
%S 2,3,5,9,11,18,63,71,162,171,407,732,821,1320,1439,1607,3008,3159,
%T 10679,13376,59294,183294
%N Numbers k such that (61*10^k + 119)/9 is prime.
%C For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 7 followed by the digits 91 is prime (see Example section).
%C a(23) > 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 67w91</a>.
%e 3 is in this sequence because (61*10^3 + 119)/9 = 179 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 691;
%e a(2) = 3, 6791;
%e a(3) = 5, 677791;
%e a(4) = 9, 6777777791;
%e a(5) = 11, 677777777791; etc.
%t Select[Range[0, 100000], PrimeQ[(61*10^# + 119)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Sep 28 2017
%E a(22) from _Robert Price_, May 23 2019