A056685 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

A056685

Numbers k such that 50*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2

0, 1, 7, 25, 65, 73, 232, 472, 539, 2773, 64714

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,3

COMMENTS

Also numbers k such that (5*10^(k+1)-23)/9 is prime.

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

LINKS

Table of n, a(n) for n=1..11.

Makoto Kamada, Prime numbers of the form 55...553.

Index entries for primes involving repunits

FORMULA

a(n) = A099416(n) - 1. - Robert Price, Nov 13 2014 [adapted by Georg Fischer, Jan 04 2021]

MATHEMATICA

Do[ If[ PrimeQ[50*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]

Select[Range[0, 3000], PrimeQ[(5 10^(# + 1) - 23) / 9] &] (* Vincenzo Librandi, Nov 14 2014 *)

PROG

(Magma) [n: n in [0..400]| IsPrime((5*10^(n+1)-23)div 9)]; // Vincenzo Librandi, Nov 14 2014

CROSSREFS

Cf. A002275, A099416.

Sequence in context: A127765 A155305 A155290 * A299262 A001296 A000970

Adjacent sequences: A056682 A056683 A056684 * A056686 A056687 A056688

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

a(11) derived from A099416 by Robert Price, Nov 13 2014

STATUS

approved