A056656 Numbers k such that 20*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 7, 10, 35, 94, 100, 127, 259, 350, 466, 644, 1010, 1177, 1216, 2441, 3760, 3805, 15616, 26458, 63116, 88544, 93496

Offset: 1

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• nonn

Comments

Also numbers k such that (2*10^(k+1)+7)/9 is prime.

Links

• Makoto Kamada, Prime numbers of the form 22...223.
• Index entries for primes involving repunits

Crossrefs

Cf. A093162 (corresponding primes), A096506.

Programs

Mathematica

Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 3 ], Print[n]], {n, 7000}]

Formula

a(n) = A096506(n) - 1.

Extensions

2441 from Rick L. Shepherd, Mar 27 2004

15616 and 26458 from Erik Branger, Jan 31 2010

63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by Ray Chandler, Feb 17 2012