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%I #12 Jan 17 2019 13:44:09
%S 2,3,5,10,21,101,133,187,209,363,747,1550,3715,3875,6022,9729,10460,
%T 12316,14421,19023,50727
%N Numbers k such that 3*10^k - 31 is prime.
%C For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 69 is prime (see Example section).
%C a(22) > 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 29w69</a>
%e 3 is in this sequence because 3*10^3 - 31 = 2969 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 269;
%e a(2) = 3, 2969;
%e a(3) = 5, 299969;
%e a(4) = 10, 29999999969;
%e a(5) = 21, 2999999999999999999969; etc.
%t Select[Range[2, 100000], PrimeQ[3*10^# - 31] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Oct 18 2017
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