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!)
A109080 Numbers n such that 5^n - 2 is prime. 18
1, 2, 14, 26, 50, 126, 144, 260, 624, 1424, 10472, 19784, 24558, 797390 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

No more terms through 18000.

No more terms through 50000. - Ryan Propper, Dec 10 2008

a(14) > 10^5. - Robert Price, Feb 03 2014

a(14) > 185000. - Tyler NeSmith, Dec 13 2020

Searched 185000 to 800000. - Paul Bourdelais, Mar 03 2021

LINKS

Table of n, a(n) for n=1..14.

F. Firoozbakht, M. F. Hasler, Variations on Euclid's formula for Perfect Numbers, JIS 13 (2010) #10.3.1

EXAMPLE

5^14 - 2 = 6103515623 is prime, hence 14 is in the sequence.

MATHEMATICA

Do[If[PrimeQ[5^n - 2], Print[n]], {n, 1, 18000}]

PROG

(PARI) for(n=1, 10^5, if(ispseudoprime(5^n-2), print1(n, ", "))) \\ Felix Fröhlich, Jun 05 2014

CROSSREFS

Sequence in context: A194411 A017545 A280268 * A295083 A180877 A032461

Adjacent sequences:  A109077 A109078 A109079 * A109081 A109082 A109083

KEYWORD

hard,nonn

AUTHOR

Ryan Propper, Jun 18 2005

EXTENSIONS

Two more terms from Ryan Propper, Dec 10 2008

a(14) from Paul Bourdelais, Mar 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 May 15 02:21 EDT 2021. Contains 343909 sequences. (Running on oeis4.)