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%I #13 May 25 2024 14:28:28
%S 0,1,4,6,24,52,60,76,94,106,136,196,283,319,486,1560,1615,1747,2188,
%T 2359,5004,5608,9301,22086,30391,76128,90588,139315
%N Numbers k such that (86*10^k + 121)/9 is prime.
%C For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 69 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 95w69</a>.
%e 4 is in this sequence because (86*10^4 + 121)/9 = 95569 is prime.
%e Initial terms and associated primes:
%e a(1) = 0, 23;
%e a(2) = 1, 109;
%e a(3) = 4, 95569;
%e a(4) = 6, 9555569;
%e a(5) = 24, 9555555555555555555555569; etc.
%t Select[Range[0, 100000], PrimeQ[(86*10^# + 121)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,3
%A _Robert Price_, Mar 02 2017
%E a(28) from _Robert Price_, Aug 30 2019
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