A296029 Numbers k such that 10^k/8 - 1 is prime.

4, 7, 8, 11, 16, 22, 28, 29, 79, 91, 170, 293, 392, 1898, 2597, 3641, 4099, 8236, 11393, 17969, 18503, 33322

Offset: 1

Author

Patrick A. Thomas, Dec 03 2017

Keywords

• nonn
• more

Comments

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

Searched up to k = 20000. - Michael B. Porter, Dec 07 2017

Searched up to k = 40000. - Michael S. Branicky, Aug 07 2024

Examples

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

Programs

Mathematica

Select[Range[2000], PrimeQ[10^#/8 - 1] &] (* Michael De Vlieger, Dec 08 2017 *)

PARI

lista(nn) = for(n=4, nn, if(ispseudoprime(10^n/8 - 1), print1(n, ", "))) \\ Iain Fox, Dec 04 2017

Extensions

a(12), a(13), a(15), a(16) from Jon E. Schoenfield, Dec 03 2017

a(14) and a(17)-a(21) from Michael B. Porter, Dec 07 2017

a(22) from Michael S. Branicky, Aug 07 2024