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%I #14 Jan 17 2019 13:44:08
%S 2,3,4,5,8,9,27,32,39,44,101,140,268,386,695,741,1155,1564,1611,4076,
%T 6083,11672,26685,50603,134720
%N Numbers k such that (2*10^k - 41)/3 is prime.
%C For n>1, numbers such that n-2 occurrences of the digit 6 followed by the digits 53 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 6w53.</a>
%e 4 is in this sequence because (2*10^4 - 41) / 3 = 6653 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 53;
%e a(2) = 3, 653;
%e a(3) = 4, 6653;
%e a(4) = 5, 66653;
%e a(5) = 8, 66666653; etc.
%t Select[Range[2, 100000], PrimeQ[(2*10^# - 41) / 3] &]
%o (PARI) is(n)=ispseudoprime((2*10^n-41)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jan 02 2017
%E a(25) from _Robert Price_, Feb 14 2018
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