login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A099416 Numbers k such that 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
1, 2, 8, 26, 66, 74, 233, 473, 540, 2774, 64715 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers k such that (5*10^k-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) = A056685(n) + 1.

MATHEMATICA

Do[ If[ PrimeQ[ 5(10^k - 1)/9 - 2], Print[k]], {k, 1, 5000}]

CROSSREFS

Cf. A002275, A056685.

Sequence in context: A216929 A100504 A327600 * A211885 A101696 A155861

Adjacent sequences:  A099413 A099414 A099415 * A099417 A099418 A099419

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Oct 14 2004

EXTENSIONS

a(11) from Robert Price, Nov 13 2014

0 removed by Georg Fischer, Jan 03 2021

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 15 03:00 EDT 2021. Contains 342974 sequences. (Running on oeis4.)