cp's OEIS Frontend

A271147 Numbers k such that (28*10^k + 113)/3 is prime.

%I A271147 #18 Jun 06 2024 23:24:16
%S A271147 0,1,2,3,4,6,9,12,16,22,23,42,52,57,63,117,119,208,266,324,481,779,
%T A271147 1244,1289,2998,5522,5599,5771,6820,12367,14737,22612,30623,31596,
%U A271147 58956,59133,138240,163709,250655,259897
%N A271147 Numbers k such that (28*10^k + 113)/3 is prime.
%C A271147 For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 3 followed by the digits 71 is prime (see Example section).
%C A271147 a(41) > 3*10^5.
%H A271147 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 93w71</a>.
%e A271147 3 is in this sequence because (28*10^3+113)/3 = 9371 is prime.
%e A271147 Initial terms and associated primes:
%e A271147 a(1) = 0, 47;
%e A271147 a(2) = 1, 131;
%e A271147 a(3) = 2, 971;
%e A271147 a(4) = 3, 9371;
%e A271147 a(5) = 4, 93371;
%e A271147 a(6) = 6, 9333371, etc.
%t A271147 Select[Range[0, 100000], PrimeQ[(28*10^# + 113)/3] &]
%o A271147 (PARI) is(n)=ispseudoprime((28*10^n + 113)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271147 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929.
%K A271147 nonn,more
%O A271147 1,3
%A A271147 _Robert Price_, Mar 31 2016
%E A271147 a(37)-a(38) from _Robert Price_, Mar 13 2020
%E A271147 a(39)-a(40) from _Robert Price_, Jun 17 2023