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%I #18 May 26 2024 14:40:21
%S 1,2,3,23,29,34,35,38,52,57,61,82,186,209,251,366,394,426,786,979,
%T 1382,2037,4557,8995,12774,19170,21828,23259,32003,41831,44999,56785,
%U 76483,97987,110468
%N Numbers k such that 7*10^k - 23 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 77 is prime (see Example section).
%C a(36) > 3*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 69w77</a>.
%e 3 is in this sequence because 7*10^3 - 23 = 6977 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 47;
%e a(2) = 2, 677;
%e a(3) = 3, 6977;
%e a(4) = 23, 699999999999999999999977;
%e a(5) = 29, 699999999999999999999999999977, etc.
%t Select[Range[0, 100000], PrimeQ[7*10^# - 23] &]
%o (PARI) is(n)=ispseudoprime(7*10^n - 23) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more
%O 1,2
%A _Robert Price_, Apr 24 2016
%E a(35) from _Robert Price_, Jul 27 2019