A056688 Numbers k such that 70*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 12, 19, 22, 30, 99, 240, 274, 924, 1066, 1368, 2064, 7162, 37962, 91855, 111705, 260197, 271756, 314563, 348723

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• nonn

Comments

Also numbers k such that (7*10^(k+1)-61)/9 is prime.

Links

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

Crossrefs

Cf. A002275, A093176, A099419.

Programs

Mathematica

Do[ If[ PrimeQ[70*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]

Formula

a(n) = A099419(n) - 1.

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