A093167 Primes of the form 20*R_k + 7, where R_k is the repunit (A002275) of length k.

7, 227, 222222227, 222222222222227, 2222222222222222222222222227, 2222222222222222222222222222222222222222222222222222222222222227

Offset: 1

Author

Rick L. Shepherd, Mar 26 2004

Keywords

• nonn
• base

Links

• Makoto Kamada, Prime numbers of the form 22...227.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056677 (corresponding k and count of digits 2 in a(n)), A099409.

Programs

Mathematica

Select[FromDigits /@ NestList["2" <> # &, "7", 10^3], PrimeQ] (* Mikk Heidemaa, Nov 18 2015 *)

PARI

list(lim)=for(i=0,lim,m=20*(10^i-1)/9 + 7;if(isprime(m),print1(m," ,"))) \\ Anders Hellström, Nov 18 2015