1,1

Also numbers n such that (2*10^(n+1)-11)/9 is a prime.

Index entries for primes involving repunits

Makoto Kamada, Factorizations of 22...221.

Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 7000}]

Cf. A091189 (corresponding primes).

Adjacent sequences: A056657 A056658 A056659 this_sequence A056661 A056662 A056663

Sequence in context: A151330 A010913 A142988 this_sequence A155610 A001541 A161940

nonn

Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 09 2000

a(8) and a(9) from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Feb 22 2004

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008