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%I #12 May 19 2024 21:56:04
%S 1,2,3,6,7,8,10,12,13,21,46,68,91,153,366,1112,1185,1222,7161,20077,
%T 33435,40747,99133,108300,136716
%N Numbers k such that 7*10^k + 69 is prime.
%C For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 69 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/prime_difficulty.txt">Search for 70w69</a>.
%e 2 is in this sequence because 7*10^2 + 69 = 769 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 139;
%e a(2) = 2, 769;
%e a(3) = 3, 7069;
%e a(4) = 6, 7000069;
%e a(5) = 7, 70000069; etc.
%t Select[Range[0, 100000], PrimeQ[7*10^# + 69] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Oct 31 2017
%E a(24)-a(25) from _Robert Price_, Jul 09 2019