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%I #15 Jan 02 2020 00:37:28
%S 4,5,9,20,21,23,39,45,63,89,94,826,994,1054,2886,4829,5880,7928,17544,
%T 47277,93226,127413
%N Numbers k such that (67*10^k - 7)/3 is prime.
%C For k>1, numbers such that the digits 22 followed by k-2 occurrences of the digit 3 followed by the digit 1 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 223w1</a>.
%e 5 is in this sequence because (67*10^5 - 7)/3 = 2233331 is prime.
%e Initial terms and primes associated:
%e a(1) = 4, 223331;
%e a(2) = 5, 2233331;
%e a(3) = 9, 22333333331;
%e a(4) = 20, 2233333333333333333331;
%e a(5) = 21, 22333333333333333333331; etc.
%t Select[Range[0, 100000], PrimeQ[(67*10^# - 7)/3] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Oct 06 2017
%E a(22) from _Robert Price_, Jan 01 2020
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