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

1,3

Comments

Terms <= 551 correspond to certified primes.

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

Links

Table of n, a(n) for n=1..15.

Henri & Renaud Lifchitz, PRP Records.

Example

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.

Mathematica

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

Prog

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

Crossrefs

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

Sequence in context: A120022 A001865 A189001 * A178685 A361767 A268254

Adjacent sequences: A087882 A087883 A087884 * A087886 A087887 A087888

Keyword

hard,nonn

Author

Donald S. McDonald, Oct 13 2003

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

Status

approved