

A087885


Numbers n such that 5^n + 2 is a prime.


13



0, 1, 3, 17, 143, 261, 551, 2285, 18731, 18995, 19751, 62067, 98051, 169727
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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..14.
Henri & Renaud Lifchitz, PRP Records.


EXAMPLE

a(3)=3 is a member 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 FrÃ¶hlich, Jun 04 2014


CROSSREFS

Cf. A051783 (3^n + 2 is prime).
Sequence in context: A120022 A001865 A189001 * A178685 A268254 A198860
Adjacent sequences: A087882 A087883 A087884 * A087886 A087887 A087888


KEYWORD

hard,nonn


AUTHOR

Donald S. McDonald, Oct 13 2003


EXTENSIONS

{551, 2285} from Ryan Propper, Jun 17 2005
{18731, 18995, 19751, 62067} found by Mike Oakes in 2003.  Alexander Adamchuk, Mar 02 2008
Edited by Ray Chandler, Jul 27 2011
98051 from Ray Chandler, Jul 28 2011
a(14) from Robert Price, Jan 16 2015


STATUS

approved



