%I #13 May 18 2024 11:32:35
%S 1,2,7,26,28,31,38,47,59,226,233,656,752,1073,1808,3646,4252,5246,
%T 24686,37952,59599
%N Numbers k such that (871*10^k - 7)/9 is prime.
%C Numbers k such that the digits 96 followed by k occurrences of the digit 7 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/prime_difficulty.txt">Search for 967w</a>.
%e 2 is in this sequence because (871*10^2 - 7)/9 = 9677 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 967;
%e a(2) = 2, 9677;
%e a(3) = 7, 967777777;
%e a(4) = 26, 9677777777777777777777777777;
%e a(5) = 28, 967777777777777777777777777777; etc.
%t Select[Range[0, 100000], PrimeQ[(871*10^# - 7)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Nov 06 2017