A099418 Numbers k such that 5*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 8, 12, 18, 26, 32, 138, 188, 222, 338, 1002, 2744, 6530, 38690, 39464, 335480, 343734

Offset

1,1

Comments

Also numbers k such that (5*10^k + 31)/9 is prime.

Links

Table of n, a(n) for n=1..17.

Makoto Kamada, Prime numbers of the form 55...559.

Index entries for primes involving repunits

Formula

a(n) = A056687(n) + 1.

Mathematica

Do[ If[ PrimeQ[ 5(10^n - 1)/9 + 4], Print[n]], {n, 0, 5000}]

Crossrefs

Cf. A002275, A093403, A056687.

Sequence in context: A046525 A235353 A057265 * A108987 A035405 A280867

Adjacent sequences: A099415 A099416 A099417 * A099419 A099420 A099421

Keyword

nonn,more

Author

Robert G. Wilson v, Oct 14 2004

Extensions

a(13)-a(15) from Kamada link by Ray Chandler, Feb 27 2012

a(16)-a(17) from Kamada data by Tyler Busby, May 03 2024

Status

approved