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

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• nonn

Comments

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

Links

• Makoto Kamada, Prime numbers of the form 22...221.
• Index entries for primes involving repunits

Crossrefs

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

Programs

Mathematica

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

Formula

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

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