A056695 Numbers k such that 80*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 3, 5, 8, 11, 71, 117, 123, 189, 243, 303, 356, 1421, 2690, 5537, 7580, 21905, 32175, 44357

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn
• more

Comments

Also numbers k such that (8*10^(k+1)-17)/9 is prime.

a(21) > 10^5. - Robert Price, May 20 2014

Links

• Makoto Kamada, Prime numbers of the form 88...887.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093171, A096846.

Programs

Mathematica

Do[ If[ PrimeQ[80*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

Formula

a(n) = A096846(n) - 1.

Extensions

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

a(18)-a(20) derived from A096846 by Robert Price, May 20 2014