A098088 Numbers k such that 6*R_k - 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 3, 4, 10, 18, 21, 22, 28, 43, 66, 121, 133, 178, 241, 454, 553, 1600, 2175, 2978, 3649, 7708, 8316, 10392, 12458, 21057, 26223, 48297, 64041, 84904, 92976, 95072, 103161, 140461, 141751, 150612, 265321

Offset

1,1

Comments

Also numbers k such that (2*10^k - 17)/3 is prime.

The terms 1600, 2175, 2978 and 3649 correspond to primes. - Joao da Silva (zxawyh66(AT)yahoo.com), Oct 03 2005

a(37) > 3*10^5, Robert Price, Oct 19 2023

Links

Table of n, a(n) for n=1..36.

Makoto Kamada, Prime numbers of the form 66...661.

Index entries for primes involving repunits

Formula

a(n) = A056658(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

Example

If n = 4 we get ((2*10^4)-17)/3 = 19983/3 = 6661, which is prime.

Mathematica

Do[ If[ PrimeQ[ 2(10^n - 1)/3 - 5], Print[n]], {n, 0, 7000}]

Crossrefs

Sequence in context: A285190 A055506 A329660 * A080500 A007661 A049891

Adjacent sequences: A098085 A098086 A098087 * A098089 A098090 A098091

Keyword

more,nonn

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004

Extensions

a(21)-a(22) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(23)-a(26) from Kamada link by Ray Chandler, Dec 23 2010

a(27) from Kamada link by Robert Price, Aug 17 2014

a(28)-a(31) from Robert Price, Aug 17 2014

a(32)-a(36) from Robert Price, Oct 19 2023

Status

approved