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

1,1

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

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 *)

Prog

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

Crossrefs

Cf. A056682 (corresponding k).

Sequence in context: A013403 A013401 A020465 * A354211 A292067 A088143

Adjacent sequences: A092477 A092478 A092479 * A092481 A092482 A092483

Keyword

nonn

Author

Rick L. Shepherd, Apr 03 2004

Status

approved