%I
%S 1,2,5,8,11,14,15,30,31,42,69,85,120,128,449,685,686,741,2165,9110,
%T 9150,11366,13154,1291,1521,33321,40758,50789,51440,65492,82509,93887
%N Numbers k such that 7*10^k  47 is prime.
%C For k>1, numbers such that the digit 6 followed by k2 occurrences of the digit 9 followed by the digits 53 is prime (see Example section).
%C a(33) > 2*10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of nearrepdigitrelated numbers</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/primedifficulty.txt">Search for 69w53</a>.
%e 2 is in this sequence because 7*10^2  47 = 653 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 23;
%e a(2) = 2, 653;
%e a(3) = 5, 699953;
%e a(4) = 8, 699999953;
%e a(5) = 11, 699999999953; etc.
%t Select[Range[1, 100000], PrimeQ[7*10^#  47] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Feb 15 2017
