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A295324 Numbers k such that (17*10^k - 89)/9 is prime. 0

%I

%S 2,3,6,12,39,45,50,75,126,282,431,459,1557,2724,3453,4058,15879,24209,

%T 31844,61760,155133

%N Numbers k such that (17*10^k - 89)/9 is prime.

%C For k > 1, numbers such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 79 is prime (see Example section).

%C a(22) > 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 18w79</a>

%e 2 is in this sequence because (17*10^2 - 89)/9 = 179 is prime.

%e Initial terms and primes associated:

%e a(1) = 2, 179;

%e a(2) = 3, 1879;

%e a(3) = 6, 1888879;

%e a(4) = 12, 1888888888879;

%e a(5) = 39, 1888888888888888888888888888888888888879; etc.

%t Select[Range[1, 100000], PrimeQ[(17*10^# - 89)/9] &]

%o (PARI) isok(k) = isprime((17*10^k - 89)/9); \\ _Michel Marcus_, Nov 20 2017

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

%K nonn,more,hard

%O 1,1

%A _Robert Price_, Nov 19 2017

%E a(21) from _Robert Price_, Jul 23 2018

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Last modified September 23 15:43 EDT 2020. Contains 337310 sequences. (Running on oeis4.)