A098959 Numbers k such that 2*10^k + 6*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 5, 6, 7, 11, 21, 30, 68, 73, 169, 176, 345, 823, 1021, 1191, 2073, 2755, 10717, 14673, 16754, 17606, 81029, 120851, 167965, 200408

Offset: 1

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Oct 21 2004

Keywords

• more
• nonn

Comments

Also numbers k such that (8*10^k - 11)/3 is prime.

a(28) > 3*10^5. - Robert Price, Jul 13 2023

Examples

			For k = 1, 2, 3, 5, 6, 7, we get 23, 263, 2663, 266663, 2666663 and 26666663 which are primes.
		

Links

• Makoto Kamada, Prime numbers of the form 266...663.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275, A101964.

Programs

Mathematica

Do[ If[ PrimeQ[(8*10^n - 11)/3], Print[n]], {n, 0, 10000}]

Formula

a(n) = A101964(n) + 1.

Extensions

a(15), a(16) & a(17) from Ray Chandler, Nov 04 2004

a(18) & a(19) from Robert G. Wilson v, Dec 17 2004

a(20)-a(23) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(24) from Kamada data by Robert Price, Jan 17 2015

a(25)-a(27) from Robert Price, Jul 13 2023