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%I #13 Jun 08 2024 15:59:06
%S 1,2,5,6,7,10,11,22,24,138,140,152,698,1180,1270,2970,3932,4972,5982,
%T 6287,8936,16014,25186,59862,130998
%N Numbers k such that 9*10^k - 19 is prime.
%C For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 9 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/prime_difficulty.txt">Search for 89w81</a>.
%e 5 is in this sequence because 9*10^5 - 19 = 899981 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 71;
%e a(2) = 2, 881;
%e a(3) = 5, 899981;
%e a(4) = 6, 8999981;
%e a(5) = 7, 89999981; etc.
%t Select[Range[1, 100000], PrimeQ[9*10^# - 19] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Oct 04 2017
%E a(25) from _Robert Price_, Jul 27 2019