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A278334 Numbers k such that (856*10^k - 1) / 9 is prime.

%I A278334 #16 May 25 2024 19:09:29
%S A278334 2,3,5,8,9,15,20,24,41,63,66,99,281,300,462,686,726,1196,1574,2543,
%T A278334 3023,5322,12161,13677,33797,137633
%N A278334 Numbers k such that (856*10^k - 1) / 9 is prime.
%C A278334 For k >= 0, numbers k such that the digits 95 followed by k occurrences of the digit 1 is prime (see Example section).
%C A278334 a(27) > 2*10^5.
%H A278334 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A278334 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 951w</a>.
%e A278334 3 is in this sequence because (856*10^3 - 1) / 9 = 95111 is prime.
%e A278334 Initial terms and associated primes:
%e A278334 a(1) = 2, 9511;
%e A278334 a(2) = 3, 95111;
%e A278334 a(3) = 5, 9511111;
%e A278334 a(4) = 8, 9511111111;
%e A278334 a(5) = 9, 95111111111; etc.
%t A278334 Select[Range[0, 100000], PrimeQ[(856*10^# - 1) / 9] &]
%o A278334 (PARI) is(n)=ispseudoprime((856*10^n - 1)/9) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A278334 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A278334 nonn,more,hard
%O A278334 1,1
%A A278334 _Robert Price_, Nov 18 2016
%E A278334 a(26) from _Robert Price_, Mar 30 2020