A056661 -id:A056661 - cp's OEIS frontend

A096845 Numbers n for which 4*R_n - 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 2, 3, 6, 9, 12, 30, 32, 183, 297, 492, 41316

Offset: 1

Labos Elemer, Jul 15 2004

Also numbers n such that (4*10^n-13)/9 is prime.

a(13) > 10^5. - Robert Price, Oct 25 2014

			n=30 means that 444444444444444444444444444443 is prime.

Makoto Kamada, Prime numbers of the form 44...443.
Index entries for primes involving repunits

Cf. A096503-A096508, A096841-A096846, A000203, A056661.

Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 1], Print[n]], {n, 5000}] (* Robert G. Wilson v, Oct 14 2004 *)
Select[Range[500],PrimeQ[FromDigits[PadLeft[{3},#,4]]]&] (* The program generates the first 11 terms of the sequence. *) (* Harvey P. Dale, Feb 10 2022 *)

a(n) = A056661(n) + 1.

a(12) from Robert Price, Oct 25 2014

A093163 Primes of the form 40*R_k + 3, where R_k is the repunit (A002275) of length k.

Original entry on oeis.org

3, 43, 443, 444443, 444444443, 444444444443, 444444444444444444444444444443, 44444444444444444444444444444443

Offset: 1

Rick L. Shepherd, Mar 26 2004

nonn

Primes of the form (4*10^k - 13)/9; the next term a(9) is a 183-digit number. - Vincenzo Librandi, Nov 16 2010

Makoto Kamada, Prime numbers of the form 44...443.
Index entries for primes involving repunits

Cf. A056661 (corresponding k), A173770.

f[n_] := (4*10^n-13)/9; Select[f[Range[50]], PrimeQ] (* Amiram Eldar, Nov 23 2018 *)
Select[Table[FromDigits[PadLeft[{3},n,4]],{n,40}],PrimeQ] (* Harvey P. Dale, Dec 19 2022 *)

cp's OEIS Frontend

A096845 Numbers n for which 4*R_n - 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.

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