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%I #11 Mar 18 2019 21:50:06
%S 1,2,3,4,10,11,15,43,51,110,164,303,824,3160,4994,8596,11717,22370,
%T 22600,65509,117205,149907
%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(23) > 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 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
%E a(21)-a(22) from _Robert Price_, Mar 18 2019
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