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A278334 Numbers k such that (856*10^k - 1) / 9 is prime. 0

%I

%S 2,3,5,8,9,15,20,24,41,63,66,99,281,300,462,686,726,1196,1574,2543,

%T 3023,5322,12161,13677,33797,137633

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

%C For k>=0, numbers such that the digits 95 followed by k occurrences of the digit 1 is prime (see Example section).

%C a(27) > 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/primedifficulty.txt">Search for 951w</a>.

%e 3 is in this sequence because (856*10^3 - 1) / 9 = 95111 is prime.

%e Initial terms and primes associated:

%e a(1) = 2, 9511;

%e a(2) = 3, 95111;

%e a(3) = 5, 9511111;

%e a(4) = 8, 9511111111;

%e a(5) = 9, 95111111111; etc.

%t Select[Range[0, 100000], PrimeQ[(856*10^# - 1) / 9] &]

%o (PARI) is(n)=ispseudoprime((856*10^n - 1)/9) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

%K nonn,more,hard,changed

%O 1,1

%A _Robert Price_, Nov 18 2016

%E a(26) from _Robert Price_, Mar 30 2020

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Last modified April 7 13:55 EDT 2020. Contains 333305 sequences. (Running on oeis4.)