cp's OEIS Frontend

A271377 Numbers k such that (28*10^k - 43)/3 is prime.

%I A271377 #18 Jun 06 2024 23:24:04
%S A271377 1,2,3,4,5,6,7,13,43,112,114,127,242,247,251,335,450,616,816,1237,
%T A271377 1448,4303,4865,5414,6427,9045,10391,12651,25071,27901,50362,58843,
%U A271377 67378,68107,262655
%N A271377 Numbers k such that (28*10^k - 43)/3 is prime.
%C A271377 For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 3 followed by the digits 19 is prime (see Example section).
%C A271377 a(36) > 3*10^5.
%H A271377 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 93w19</a>.
%e A271377 3 is in this sequence because (28*10^3 - 43)/3 = 9319 is prime.
%e A271377 Initial terms and associated primes:
%e A271377 a(1) = 1, 79;
%e A271377 a(2) = 2, 919;
%e A271377 a(3) = 3, 9319;
%e A271377 a(4) = 4, 93319;
%e A271377 a(5) = 5, 933319, etc.
%t A271377 Select[Range[0, 100000], PrimeQ[(28*10^# - 43)/3] &]
%o A271377 (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((28*10^n - 43)/3), print1(n, ", "))); \\ _Altug Alkan_, Apr 05 2016
%Y A271377 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271377 nonn,more
%O A271377 1,2
%A A271377 _Robert Price_, Apr 05 2016
%E A271377 a(35) from _Robert Price_, Jul 02 2023