A087885 Numbers k such that 5^k + 2 is a prime.

0, 1, 3, 17, 143, 261, 551, 2285, 18731, 18995, 19751, 62067, 98051, 169727, 442281

Offset: 1

Author

Donald S. McDonald, Oct 13 2003

Keywords

• hard
• nonn

Comments

Terms <= 551 correspond to certified primes.

a(15) > 2*10^5. - Robert Price, Jan 16 2015

Examples

			a(3)=3 is a term because 5^3 + 2 = 127 is a prime.
5^17 + 2 = 762939453127 is prime, hence 17 is a term.
		

Links

• Henri & Renaud Lifchitz, PRP Records.

Crossrefs

Cf. A051783 (3^n + 2 is prime).

Programs

Mathematica

Do[If[PrimeQ[5^n + 2], Print[n]], {n, 1, 10000}] (* Ryan Propper, Jun 17 2005 *)

PARI

for(n=0, 10^5, if(ispseudoprime(5^n+2), print1(n, ", "))) \\ Felix Fröhlich, Jun 04 2014

Extensions

a(7)-a(8) from Ryan Propper, Jun 17 2005

a(9)-a(12) found by Mike Oakes in 2003. - Alexander Adamchuk, Mar 02 2008

Edited by Ray Chandler, Jul 27 2011

a(13) from Ray Chandler, Jul 28 2011

a(14) from Robert Price, Jan 16 2015

a(15) from Paul Bourdelais, Jan 28 2021