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%I #18 May 03 2024 18:55:52
%S 3,4,7,9,48,52,105,153,1783,1999,2787,4276,13693,14067,19143,20431,
%T 25933,26206,29062,40462,40791,58738,81709
%N Numbers k such that (8*10^k - 611)/9 is prime.
%C For k > 1, numbers k such that k-2 occurrences of the digit 8 followed by the digits 21 is prime (see Example section).
%C a(24) > 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 8w21</a>.
%e 3 is in this sequence because (8*10^3 - 611)/9 = 821 is prime.
%e Initial terms and associated primes:
%e a(1) = 3, 821;
%e a(2) = 4, 8821;
%e a(3) = 7, 8888821;
%e a(4) = 9, 888888821; etc.
%t Select[Range[2, 100000], PrimeQ[(8*10^# - 611)/9] &]
%o (PARI) isok(k) = isprime((8*10^k - 611)/9); \\ _Michel Marcus_, Nov 22 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 21 2017
%E Incorrect a(1)=2 removed by _Georg Fischer_, Jun 26 2020
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