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

Author

Rick L. Shepherd, Mar 27 2004

Keywords

• nonn

Comments

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

Links

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

Crossrefs

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

Programs

Mathematica

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

Formula

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

Extensions

Edited by Ray Chandler, Mar 06 2012