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 A295393 Numbers k such that 5*10^k + 23 is prime. 0

%I

%S 1,2,3,4,10,11,15,43,51,110,164,303,824,3160,4994,8596,11717,22370,

%T 22600,65509

%N Numbers k such that 5*10^k + 23 is prime.

%C For k > 1, numbers such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 23 is prime (see Example section).

%C a(21) > 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 50w23</a>

%e 2 is in this sequence because 5*10^2 + 23 = 523 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 73;

%e a(2) = 2, 523;

%e a(3) = 3, 5023;

%e a(4) = 4, 50023;

%e a(5) = 10, 50000000023; etc.

%t Select[Range[0, 100000], PrimeQ[5*10^# + 23] &]

%o (PARI) isok(k) = isprime(5*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 February 20 21:29 EST 2019. Contains 320350 sequences. (Running on oeis4.)