A056660 Numbers k such that 20*R_k + 1 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 17, 99, 120, 243, 545, 630, 1493, 2565, 8087

Offset

1,1

Comments

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

Links

Table of n, a(n) for n=1..10.

Makoto Kamada, Prime numbers of the form 22...221.

Index entries for primes involving repunits

Formula

a(n) = A084832(n) - 1. - Robert Price, Jan 30 2015

Mathematica

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

Crossrefs

Cf. A002275, A084832, A091189 (corresponding primes).

Sequence in context: A010913 A142988 A356267 * A155610 A001541 A322242

Adjacent sequences: A056657 A056658 A056659 * A056661 A056662 A056663

Keyword

nonn

Author

Robert G. Wilson v, Aug 09 2000

Extensions

a(8) and a(9) from Rick L. Shepherd, Feb 22 2004

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

Status

approved