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

1,2

Comments

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

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

Links

Table of n, a(n) for n=1..27.

Makoto Kamada, Prime numbers of the form 266...663.

Index entries for primes involving repunits.

Formula

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

Example

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

Mathematica

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

Crossrefs

Cf. A002275, A101964.

Sequence in context: A062084 A331394 A273524 * A071251 A077571 A101730

Adjacent sequences: A098956 A098957 A098958 * A098960 A098961 A098962

Keyword

more,nonn

Author

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

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

Status

approved