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A093167
Primes of the form 20*R_k + 7, where R_k is the repunit (A002275) of length k.
2
7, 227, 222222227, 222222222222227, 2222222222222222222222222227, 2222222222222222222222222222222222222222222222222222222222222227
OFFSET
1,1
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.
Sequence in context: A372967 A142606 A123832 * A157693 A221029 A221495
KEYWORD
nonn,base
AUTHOR
Rick L. Shepherd, Mar 26 2004
STATUS
approved