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%I #13 Jan 17 2019 13:44:09
%S 1,2,4,7,8,26,64,116,123,157,178,288,328,1730,2712,3244,3865,7766,
%T 8792,9512,14917,33912,39058,57997,120306,150675,171306,173467,175965
%N Numbers k such that (13*10^k - 37)/3 is prime.
%C For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 21 is prime (see Example section).
%C a(30) > 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 43w21.</a>
%e 4 is in this sequence because (13*10^4 - 37)/3 = 43321 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 31;
%e a(2) = 2, 421;
%e a(3) = 4, 43321;
%e a(4) = 7, 43333321;
%e a(5) = 8, 433333321; etc.
%t Select[Range[2, 100000], PrimeQ[(13*10^# - 37)/3] &]
%o (PARI) isok(n) = isprime((13*10^n - 37)/3); \\ _Altug Alkan_, Aug 21 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Aug 19 2017
%E a(25)-a(29) from _Robert Price_, Jan 01 2019
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