A056688 -id:A056688 - cp's OEIS frontend

A093176 Primes of the form 70*R_k + 1, where R_k is the repunit (A002275) of length k.

71, 7777777777771, 77777777777777777771, 77777777777777777777771, 7777777777777777777777777777771, 7777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777771

Offset: 1

Rick L. Shepherd, Mar 27 2004

nonn

Primes of the form (7*10^k - 61)/9. - Vincenzo Librandi, Nov 16 2010

The next term (a(7)) has 241 digits. - Harvey P. Dale, Jul 01 2022

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

Cf. A002275, A056688 (corresponding k), A099419.

Select[Table[FromDigits[PadLeft[{1},n,7]],{n,100}],PrimeQ] (* Harvey P. Dale, Jul 01 2022 *)

a(n) = (70*10^A056688(n) - 61)/9 = (7*10^A099419(n) - 61)/9.

Edited by Ray Chandler, Mar 06 2012

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

Original entry on oeis.org

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

Offset: 1

Robert G. Wilson v, Oct 14 2004

nonn

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

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

Cf. A002275, A056688, A093176.

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

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

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

cp's OEIS Frontend

A093176 Primes of the form 70*R_k + 1, where R_k is the repunit (A002275) of length k.

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