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

Author

Ryan Propper, Jun 18 2005

Keywords

• hard
• nonn

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

Examples

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

Links

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

Programs

Mathematica

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

PARI

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

Extensions

Two more terms from Ryan Propper, Dec 10 2008

a(14) from Paul Bourdelais, Mar 03 2021