A296029 - OEIS

%I #29 Aug 07 2024 09:45:51

%S 4,7,8,11,16,22,28,29,79,91,170,293,392,1898,2597,3641,4099,8236,

%T 11393,17969,18503,33322

%N Numbers k such that 10^k/8 - 1 is prime.

%C Equivalently, numbers k such that 125*10^(k-3) - 1 is prime.

%C Searched up to k = 20000. - _Michael B. Porter_, Dec 07 2017

%C Searched up to k = 40000. - _Michael S. Branicky_, Aug 07 2024

%e 1249, 1249999, and 12499999 are prime, so 4, 7, and 8 are in the sequence.

%t Select[Range[2000], PrimeQ[10^#/8 - 1] &] (* _Michael De Vlieger_, Dec 08 2017 *)

%o (PARI) lista(nn) = for(n=4, nn, if(ispseudoprime(10^n/8 - 1), print1(n, ", "))) \\ _Iain Fox_, Dec 04 2017

%K nonn,more

%O 1,1

%A _Patrick A. Thomas_, Dec 03 2017

%E a(12), a(13), a(15), a(16) from _Jon E. Schoenfield_, Dec 03 2017

%E a(14) and a(17)-a(21) from _Michael B. Porter_, Dec 07 2017

%E a(22) from _Michael S. Branicky_, Aug 07 2024