A056685 - OEIS

A056685

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

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

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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

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

KEYWORD

hard,nonn

AUTHOR

EXTENSIONS

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

STATUS

approved