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%I #14 Feb 09 2020 02:42:16
%S 1,2,14,26,53,86,133,206,245,323,362,872,4562,12577,14294,18425,21713,
%T 27728,87368,106772
%N Numbers k such that (115*10^k - 7)/9 is prime.
%C For k > 0, numbers such that the digits 12 followed by k occurrences of the digit 7 is prime (see Example section).
%C a(21) > 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 127w</a>.
%e 2 is in this sequence because (115*10^2 - 7)/9 = 1277 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 127;
%e a(2) = 2, 1277;
%e a(3) = 14, 1277777777777777;
%e a(4) = 26, 1277777777777777777777777777; etc.
%t Select[Range[0, 100000], PrimeQ[(115*10^# - 7)/9] &]
%t Select[Range[87500],PrimeQ[FromDigits[PadRight[{1,2},#,7]]]&]-2 (* _Harvey P. Dale_, May 27 2018 *)
%o (PARI) isok(k) = isprime((115*10^k - 7)/9); \\ _Michel Marcus_, Nov 14 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Nov 13 2017
%E a(20) from _Robert Price_, Feb 08 2020
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