A092480 Primes of the form 40*R_k + 7, where R_k is the repunit (A002275) of length k.

7, 47, 4447, 4444444447, 44444444444444444447, 44444444444444444444444447

Offset: 1

Author

Rick L. Shepherd, Apr 03 2004

Keywords

• nonn

Comments

Primes of the form ((4*10^k - 31)/9) + 6. - Vincenzo Librandi, Dec 13 2011

The next term has 722 digits. - Harvey P. Dale, Jan 19 2020

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..7
• Makoto Kamada, Prime numbers of the form 44...447.
• Index entries for primes involving repunits

Crossrefs

Cf. A056682 (corresponding k).

Programs

Magma

[a: n in [1..720] | IsPrime(a) where a is ((4*10^n-31) div 9)+6 ]; // Vincenzo Librandi, Dec 13 2011

Mathematica

Select[Table[(((4*10^n-31)/ 9)+6),{n,1,800}],PrimeQ] (* Vincenzo Librandi, Dec 13 2011 *)
Select[Table[FromDigits[PadLeft[{7},n,4]],{n,30}],PrimeQ] (* Harvey P. Dale, Jan 19 2020 *)