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%I #14 Sep 08 2022 08:46:18
%S 2,3,5,6,9,14,18,21,24,27,29,83,513,555,750,843,1118,4494,5886,6968,
%T 9519,12290,15779,76536,76818,90371
%N Numbers k such that (7*10^k - 313) / 9 is prime.
%C For n>1, numbers such that n-2 occurrences of the digit 7 followed by the digits 43 is prime (see Example section).
%C a(27) > 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 7w43.</a>
%e 3 is in this sequence because (7*10^3 - 313) / 9 = 743 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 43;
%e a(2) = 3, 743;
%e a(3) = 5, 77743;
%e a(4) = 6, 777743;
%e a(5) = 9, 777777743; etc.
%t Select[Range[0, 100000], PrimeQ[(7*10^# - 313) / 9] &]
%o (PARI) isok(n) = isprime((7*10^n - 313) / 9); \\ _Michel Marcus_, Nov 27 2016
%o (Magma) [n: n in [0..500] | IsPrime((7*10^n-313) div 9)]; // _Vincenzo Librandi_, Nov 27 2016
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 26 2016
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