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%I #9 Aug 22 2019 20:40:09
%S 0,2,3,9,11,26,39,42,58,75,105,167,279,561,2152,2378,3774,5001,5736,
%T 9470,13419,14535,32291,89800,146421
%N Numbers k such that 8*10^k + 81 is prime.
%C For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section).
%C a(26) > 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/primedifficulty.txt">Search for 80w81</a>.
%e 3 is in this sequence because 8*10^3 + 81 = 8081 is prime.
%e Initial terms and primes associated:
%e a(1) = 0, 89;
%e a(2) = 2, 881;
%e a(3) = 3, 8081;
%e a(4) = 9, 8000000081;
%e a(5) = 11, 800000000081; etc.
%t Select[Range[0, 100000], PrimeQ[(8*10^# + 81] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, May 29 2017
%E a(25) from _Robert Price_, Aug 22 2019
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