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%I #12 Jan 17 2019 13:44:08
%S 1,2,5,9,12,14,36,61,79,96,121,126,131,149,175,204,359,404,533,782,
%T 1869,1950,2712,2915,66551
%N Numbers k such that (13*10^k - 73) / 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 09 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/primedifficulty.txt">Search for 43w09.</a>
%e 5 is in this sequence because (13*10^5 - 73) / 3 = 433309 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 19;
%e a(2) = 2, 409;
%e a(3) = 5, 433309;
%e a(4) = 9, 4333333309;
%e a(5) = 12, 4333333333309; etc.
%t Select[Range[0, 100000], PrimeQ[(13*10^# - 73) / 3] &]
%o (PARI) is(n)=ispseudoprime((13*10^n - 73)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Dec 07 2016
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