A297422 Numbers k such that 4*10^(2k) + 2*(10^k) + 1 is prime.

0, 1, 3, 5, 10, 196, 251, 310, 415, 1107, 2422

Offset: 1

Author

Patrick A. Thomas, Dec 30 2017

Keywords

• nonn
• more

Comments

Numbers of this form divide 8*(10^3k)-1.

a(12) > 50000. - Robert Price, Apr 11 2018

Examples

			7, 421, 4002001, and 40000200001 are prime, while 40201=7*5743, 400020001=31*12903871, etc.
		

Programs

Maple

select(k->isprime(4*10^(2*k)+2*(10^k)+1),[$0..1000]); # Muniru A Asiru, Apr 11 2018

Mathematica

Select[Range[1500], PrimeQ[4 (10^(2 #)) + 2 (10^#) + 1] &] (* Michael De Vlieger, Dec 30 2017 *)

PARI

isok(k) = isprime(4*10^(2*k)+2*(10^k)+1); \\ Michel Marcus, Dec 30 2017

Extensions

a(6)-a(10) from Michael De Vlieger, Dec 30 2017

a(11) from Jon E. Schoenfield, Jan 01 2018