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%I #17 May 18 2024 13:37:58
%S 2,8,14,30,43,61,79,84,109,121,203,211,215,224,318,392,1061,1304,1535,
%T 1559,6989,9201,13485,35462,39408,43136,70680,126600,132884,171535
%N Numbers k such that 7*10^k - 99 is prime.
%C For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 01 is prime (see Example section).
%C a(31) > 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 69w01</a>.
%e 2 is in this sequence because 7*10^2 - 99 = 601 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 601;
%e a(2) = 8, 699999901;
%e a(3) = 14, 699999999999901;
%e a(4) = 30, 6999999999999999999999999999901;
%e a(5) = 43, 69999999999999999999999999999999999999999901; etc.
%t Select[Range[2, 100000], PrimeQ[7*10^# - 99] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 05 2017
%E a(28)-a(30) from _Robert Price_, Jun 29 2019