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%I #15 Jan 17 2019 13:44:09
%S 2,3,5,8,30,50,52,120,160,215,260,306,537,539,10690,13888,15495,16874,
%T 18448,22233,59394,103749
%N Numbers k such that 2*10^k - 69 is prime.
%C For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 31 is prime (see Example section).
%C a(23) > 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 19w31.</a>
%e 3 is in this sequence because 2*10^3 - 69 = 1931 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 131;
%e a(2) = 3, 1931;
%e a(3) = 5, 199931;
%e a(4) = 8, 199999931;
%e a(5) = 30, 1999999999999999999999999999931; etc.
%t Select[Range[1, 100000], PrimeQ[2*10^# - 69] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jul 18 2017
%E a(22) from _Robert Price_, Mar 22 2018
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