login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A109080 Numbers n such that 5^n - 2 is prime. 19

%I #30 Mar 03 2021 09:37:20

%S 1,2,14,26,50,126,144,260,624,1424,10472,19784,24558,797390

%N Numbers n such that 5^n - 2 is prime.

%C No more terms through 18000.

%C No more terms through 50000. - _Ryan Propper_, Dec 10 2008

%C a(14) > 10^5. - _Robert Price_, Feb 03 2014

%C a(14) > 185000. - _Tyler NeSmith_, Dec 13 2020

%C Searched 185000 to 800000. - _Paul Bourdelais_, Mar 03 2021

%H F. Firoozbakht, M. F. Hasler, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL13/Hasler/hasler2.html">Variations on Euclid's formula for Perfect Numbers</a>, JIS 13 (2010) #10.3.1

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

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

%o (PARI) for(n=1, 10^5, if(ispseudoprime(5^n-2), print1(n, ", "))) \\ _Felix Fröhlich_, Jun 05 2014

%K hard,nonn

%O 1,2

%A _Ryan Propper_, Jun 18 2005

%E Two more terms from _Ryan Propper_, Dec 10 2008

%E a(14) from _Paul Bourdelais_, Mar 03 2021

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 14:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)