A099422 Numbers k such that 8*R_k - 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 5, 8, 9, 15, 51, 71, 77, 224, 296, 315, 2090, 2906, 3395, 3882, 5114, 6056, 7254, 7995, 18173, 18971, 35006, 69674, 175428, 253313

Offset

1,2

Comments

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

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

Links

Table of n, a(n) for n=1..27.

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

Index entries for primes involving repunits

Formula

a(n) = A056694(n) + 1.

Mathematica

Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 5], Print[n]], {n, 1, 5000}]

Prog

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

Crossrefs

Cf. A002275, A056664, A056694, A093166.

Sequence in context: A168154 A278334 A120057 * A294913 A056903 A229139

Adjacent sequences: A099419 A099420 A099421 * A099423 A099424 A099425

Keyword

nonn,changed

Author

Robert G. Wilson v, Oct 14 2004

Extensions

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

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

a(1)=0 removed by Georg Fischer, Jan 03 2021

a(26)-a(27) from Kamada data by Tyler Busby, Apr 16 2024

Status

approved