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%I #20 Jan 17 2019 13:44:10
%S 3,5,17,20,21,107,176,207,2115,2156,2571,4082,5291,7148,7525,16857,
%T 23245,23347,31307,35107,54527
%N Numbers k such that (2*10^k - 197)/3 is prime.
%C For k > 1, numbers such that k-2 occurrences of the digit 6 followed by the digits 01 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 6w01</a>
%e 3 is in this sequence because (2*10^3 - 197)/3 = 601 is prime.
%e Initial terms and primes associated:
%e a(1) = 3, 601;
%e a(2) = 5, 66601;
%e a(3) = 17, 66666666666666601;
%e a(4) = 20, 66666666666666666601;
%e a(5) = 21, 666666666666666666601; etc.
%t Select[Range[2, 100000], PrimeQ[(2*10^# - 197)/3] &]
%o (PARI) isok(k) = isprime((2*10^k - 197)/3); \\ _Michel Marcus_, Nov 22 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 21 2017
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