cp's OEIS Frontend

A270613 Numbers k such that (68*10^k + 7)/3 is prime.

%I A270613 #19 May 26 2024 16:02:05
%S A270613 1,2,3,4,7,10,24,25,29,34,35,37,46,49,88,103,290,381,484,696,751,886,
%T A270613 999,1750,5062,6214,9740,12558,16551,24674,28600,37427,48032,61991,
%U A270613 70148,72516,99441,179656
%N A270613 Numbers k such that (68*10^k + 7)/3 is prime.
%C A270613 Numbers k such that the digits 22 followed by k-1 occurrences of the digit 6 followed by the digit 9 is prime (see Example section).
%C A270613 a(39) > 3*10^5.
%H A270613 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 226w9</a>.
%e A270613 3 is in this sequence because (68*10^3+7)/3 = 22669 is prime.
%e A270613 Initial terms and associated primes:
%e A270613 a(1) = 1, 229;
%e A270613 a(2) = 2, 2269;
%e A270613 a(3) = 3, 22669;
%e A270613 a(4) = 4, 226669;
%e A270613 a(5) = 7, 226666669, etc.
%t A270613 Select[Range[0, 100000], PrimeQ[(68*10^# + 7)/3] &]
%o A270613 (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((68*10^n + 7)/3), print1(n, ", "))); \\ _Altug Alkan_, Mar 20 2016
%Y A270613 Cf. A056654, A268448, A269303, A270339.
%K A270613 nonn,more
%O A270613 1,2
%A A270613 _Robert Price_, Mar 20 2016
%E A270613 a(38) from _Robert Price_, Jan 16 2020