%I #11 Jun 10 2024 23:37:12
%S 1,2,3,6,12,14,29,31,35,58,77,79,108,122,154,895,1045,1763,3243,3605,
%T 9446,16119,19238,28562,87726,89413,107804,136303
%N Numbers k such that (25*10^k + 161)/3 is prime.
%C For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 87 is prime (see Example section).
%C a(29) > 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 83w87</a>.
%e 3 is in this sequence because (25*10^3 + 161) / 3 = 8387 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 137;
%e a(2) = 2, 887;
%e a(3) = 3, 8387;
%e a(4) = 6, 8333387;
%e a(5) = 12, 8333333333387; etc.
%t Select[Range[0, 100000], PrimeQ[(25*10^# + 161) / 3] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Jan 14 2017
%E a(27)-a(28) from _Robert Price_, Sep 30 2019