A056678 Numbers k such that 20*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400, 10390, 12104, 19615

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (2*10^(k+1)+61)/9 is prime.

a(14) > 10^5. - Robert Price, Nov 01 2014

Links

• Makoto Kamada, Prime numbers of the form 22...229.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093401 (corresponding primes), A099410.

Programs

Mathematica

Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]

Formula

a(n) = A099410(n+1) - 1. - Robert Price, Nov 01 2014

Extensions

Two more terms from Rick L. Shepherd, Mar 30 2004

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

a(11)-a(13) derived from A099410 by Robert Price, Nov 01 2014