A056694 Numbers k such that 80*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 4, 7, 8, 14, 50, 70, 76, 223, 295, 314, 2089, 2905, 3394, 3881, 5113, 6055, 7253, 7994, 18172, 18970, 35005, 69673

Offset

1,3

Comments

Also numbers k such that (8*10^(k+1)-53)/9 is prime.

a(26) > 10^5. - Robert Price, Oct 31 2014

Links

Table of n, a(n) for n=1..25.

Makoto Kamada, Prime numbers of the form 88...883.

Index entries for primes involving repunits

Formula

a(n) = A099422(n) - 1. [adapted by Georg Fischer, Jan 04 2021]

Mathematica

Do[ If[ PrimeQ[80*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]

Prog

(Magma) [n: n in [0..500] | IsPrime((8*10^(n+1)-53) div 9)]; // Vincenzo Librandi, Nov 01 2014

Crossrefs

Cf. A002275, A093166, A099422.

Sequence in context: A015893 A018430 A018265 * A090606 A241548 A373801

Adjacent sequences: A056691 A056692 A056693 * A056695 A056696 A056697

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(22)-a(25) from Robert Price, Oct 31 2014

Status

approved