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%I #11 Jun 02 2024 09:29:15
%S 1,3,6,13,31,58,76,96,147,174,306,532,1177,3019,5790,5952,7489,10269,
%T 10936,20209,28188,55645,71068,89640
%N Numbers k such that (86*10^k - 311)/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 21 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 95w21</a>.
%e 3 is in this sequence because (86*10^3 - 311)/9 = 9521 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 61;
%e a(2) = 3, 9521;
%e a(3) = 6, 9555521;
%e a(4) = 13, 95555555555521;
%e a(5) = 31, 95555555555555555555555555555521; etc.
%t Select[Range[1, 100000], PrimeQ[(86*10^# - 311)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Oct 19 2017
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