A271109 - OEIS

%I #44 Jun 02 2024 09:05:45

%S 2,3,5,6,8,11,26,33,35,41,69,73,204,230,295,381,392,537,776,1187,2187,

%T 2426,4182,4589,5841,6107,11513,13431,28901,56256,65203,66613,82085,

%U 91707,126871,140281

%N Numbers k such that (5 * 10^k - 119)/3 is prime.

%C For k > 1, numbers k such that the digit 1 followed by k - 2 occurrences of the digit 6 followed by the digits 27 is prime (see Example section).

%C a(37) > 2*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 16w27</a>.

%e 3 is in this sequence because (5*10^3 - 119)/3 = 1627 is prime.

%e Initial terms and associated primes:

%e a(1) = 2, 127;

%e a(2) = 3, 1627;

%e a(3) = 5, 166627;

%e a(4) = 6, 1666627;

%e a(5) = 8, 166666627, etc.

%t Select[Range[10^5], PrimeQ[(5 * 10^# - 119)/3] &]

%o (PARI) is(n)=ispseudoprime((5*10^n-119)/3) \\ _Charles R Greathouse IV_, Jun 13 2017

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

%K nonn,more

%O 1,1

%A _Robert Price_, Apr 05 2016

%E a(35)-a(36) from _Robert Price_, Mar 29 2018