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%I #16 Jan 17 2019 13:44:09
%S 2,3,4,7,9,15,57,63,147,178,202,697,713,952,1861,7433,14311,16737,
%T 29369,72723,121543
%N Numbers k such that (10^k - 79)/3 is prime.
%C For k > 1, numbers such that k - 2 occurrences of the digit 3 followed by the digits 07 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 3w07</a>
%e 3 is in this sequence because (10^3 - 79)/3 = 307 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 7;
%e a(2) = 3, 307;
%e a(3) = 4, 3307;
%e a(4) = 7, 3333307;
%e a(5) = 9, 333333307; etc.
%t Select[Range[2, 100000], PrimeQ[(10^# - 79)/3] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Oct 17 2017
%E a(21) from _Robert Price_, Jan 09 2018
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