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A295400 Numbers k such that 3*10^k - 23 is prime. 0

%I

%S 1,2,5,8,14,18,20,36,68,224,252,563,780,2430,3150,7919,11092,14020,

%T 14908,58032

%N Numbers k such that 3*10^k - 23 is prime.

%C For k > 1, numbers such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).

%C a(21) > 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 29w77</a>

%e 2 is in this sequence because 3*10^2 - 23 = 277 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 7;

%e a(2) = 2, 277;

%e a(3) = 5, 299977;

%e a(4) = 8, 299999977;

%e a(5) = 14, 299999999999977; etc.

%t Select[Range[1, 100000], PrimeQ[3*10^# - 23] &]

%o (PARI) isok(k) = isprime(3*10^k - 23); \\ _Michel Marcus_, Nov 22 2017

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

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Nov 21 2017

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Last modified December 14 12:04 EST 2019. Contains 329979 sequences. (Running on oeis4.)