A099419 Numbers k such that 7*R_k - 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 13, 20, 23, 31, 100, 241, 275, 925, 1067, 1369, 2065, 7163, 37963, 91856, 111706, 260198, 271757, 314564, 348724

Offset: 1

Author

Robert G. Wilson v, Oct 14 2004

Keywords

• nonn

Comments

Also numbers k such that (7*10^k - 61)/9 is prime.

Links

• Makoto Kamada, Prime numbers of the form 77...771.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A056688, A093176.

Programs

Mathematica

Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 6], Print[n]], {n, 0, 7000}]

Formula

a(n) = A056688(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

Extensions

a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(14)-a(15) from Erik Branger, Dec 19 2009

a(16) from Erik Branger, Jan 29 2011

a(17) from Erik Branger, Nov 26 2011

a(18) from Erik Branger, Mar 05 2012

a(19)-a(20) from Erik Branger, Sep 23 2016