A056685 -id:A056685 - cp's OEIS frontend

A093164 Primes of the form 50*R_k + 3, where R_k is the repunit (A002275) of length k.

3, 53, 55555553, 55555555555555555555555553, 555555555555555555555555555555555555555555555555555555555555555553, 55555555555555555555555555555555555555555555555555555555555555555555555553

Offset: 1

Rick L. Shepherd, Mar 26 2004

nonn

Primes of the form (5*10^k - 23)/9. - Vincenzo Librandi, Nov 16 2010

The next term (a(7)) has 233 digits. - Harvey P. Dale, Mar 04 2024

Makoto Kamada, Prime numbers of the form 55...553.
Index entries for primes involving repunits

Cf. A002275, A056685 (corresponding k).

Select[(5(10^Range[74])-23)/9,PrimeQ] (* Paul F. Marrero Romero, Oct 20 2023 *)
Select[Table[FromDigits[PadLeft[{3},n,5]],{n,75}],PrimeQ] (* Harvey P. Dale, Mar 04 2024 *)

A099416 Numbers k such that 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

1, 2, 8, 26, 66, 74, 233, 473, 540, 2774, 64715, 102492, 141594

Offset: 1

Robert G. Wilson v, Oct 14 2004

nonn

Also numbers k such that (5*10^k-23)/9 is prime.

a(12) > 10^5. - Robert Price, Nov 13 2014

Makoto Kamada, Prime numbers of the form 55...553.
Index entries for primes involving repunits

Cf. A002275, A056685.

Do[ If[ PrimeQ[ 5(10^k - 1)/9 - 2], Print[k]], {k, 1, 5000}]

a(n) = A056685(n) + 1.

a(11) from Robert Price, Nov 13 2014
0 removed by Georg Fischer, Jan 03 2021
a(12)-a(13) from Kamada data by Tyler Busby, May 03 2024

cp's OEIS Frontend

A093164 Primes of the form 50*R_k + 3, where R_k is the repunit (A002275) of length k.

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