A056696 Numbers k such that 90*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 4, 6, 32, 44, 104, 196, 198, 280, 300, 316, 1106, 1656, 3394, 35924, 37596, 64304, 80138, 221630

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• nonn

Comments

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

Links

• Makoto Kamada, Prime numbers of the form 99...991.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093177, A095714.

Programs

Mathematica

Do[ If[ PrimeQ[90*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]

Formula

a(n) = A095714(n) - 1.

Extensions

a(15) - a(16) from Jason Earls, Jan 07 2008

a(17) - a(19) from Alexander Gramolin, May 13 2011

Edited by Ray Chandler, Feb 26 2012