A109080 Numbers n such that 5^n - 2 is prime.

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

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