cp's OEIS Frontend

A271109 Numbers k such that (5 * 10^k - 119)/3 is prime.

%I A271109 #44 Jun 02 2024 09:05:45
%S A271109 2,3,5,6,8,11,26,33,35,41,69,73,204,230,295,381,392,537,776,1187,2187,
%T A271109 2426,4182,4589,5841,6107,11513,13431,28901,56256,65203,66613,82085,
%U A271109 91707,126871,140281
%N A271109 Numbers k such that (5 * 10^k - 119)/3 is prime.
%C A271109 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 A271109 a(37) > 2*10^5.
%H A271109 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 16w27</a>.
%e A271109 3 is in this sequence because (5*10^3 - 119)/3 = 1627 is prime.
%e A271109 Initial terms and associated primes:
%e A271109 a(1) = 2, 127;
%e A271109 a(2) = 3, 1627;
%e A271109 a(3) = 5, 166627;
%e A271109 a(4) = 6, 1666627;
%e A271109 a(5) = 8, 166666627, etc.
%t A271109 Select[Range[10^5], PrimeQ[(5 * 10^# - 119)/3] &]
%o A271109 (PARI) is(n)=ispseudoprime((5*10^n-119)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271109 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271109 nonn,more
%O A271109 1,1
%A A271109 _Robert Price_, Apr 05 2016
%E A271109 a(35)-a(36) from _Robert Price_, Mar 29 2018