A093167 - OEIS

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

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OFFSET

1,1

LINKS

MATHEMATICA

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

PROG

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

CROSSREFS

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

KEYWORD

nonn,base

AUTHOR

STATUS

approved