A295988 Numbers k such that (10^k)/2 - 1 is prime.

3, 4, 5, 7, 15, 55, 211, 391, 595, 3461, 5029, 5220, 5333, 8073, 15797, 16132, 21457, 29283, 78791, 85143, 179973, 211030, 445774, 464844, 511057

Offset: 1

Author

Patrick A. Thomas, Dec 02 2017

Keywords

• nonn
• more

Examples

			499, 4999, 49999, 4999999 are prime, while 4, 49, 499999, 49999999 are composite.
		

Crossrefs

Cf. A056712, A093945.

Programs

Mathematica

Select[Range[10^3], PrimeQ[10^#/2 - 1] &] (* Michael De Vlieger, Dec 02 2017 *)

PARI

isok(n) = isprime(10^n/2 - 1); \\ Michel Marcus, Dec 02 2017

Formula

a(n) = 1 + A056712(n). - Omar E. Pol, Dec 02 2017

Extensions

a(7)-a(25) from Michel Marcus and Omar E. Pol, Dec 02 2017