A133127 Numbers k with the property that k followed by k 7's is prime.

1, 2, 4, 32, 122, 388, 1580, 2302, 3436, 3866, 16993, 35263, 42601, 51251

Offset: 1

Author

Ray G. Opao, Sep 19 2007

Keywords

• nonn
• base
• more

Comments

The numbers formed by a(1) to a(8) are certified prime and those formed by a(9) and a(10) are prp's.

a(11) > 10000.

Examples

			2 is a term since 277 is prime.
		

Crossrefs

Cf. A084428.

Programs

Mathematica

Select[Range[1600], PrimeQ[#*10^# + 7*(10^# - 1)/9] &] (* Amiram Eldar, Jan 27 2021 *)

PARI

isok(n) = { ss = Str(7*(10^n-1)/9); ispseudoprime(eval(concat(Str(n), ss)));} \\ Michel Marcus, Aug 09 2013

Extensions

a(11) from Amiram Eldar, Jan 27 2021

a(12)-a(14) from Michael S. Branicky, Jun 06 2024