OFFSET
1,1
COMMENTS
Note that if n > 2 and n+1 is prime then (by Fermat's theorem) n+1 divides 3^n+n.
If it exists, a(6) > 100000. - Hugo Pfoertner, Mar 01 2024
MATHEMATICA
Do[ If[ PrimeQ[ 3^n + n ], Print[ n ] ], {n, 0, 3000} ]
v={2}; Do[If[EvenQ[n]&&Mod[n, 3]!=0&&!PrimeQ[n+1]&&PrimeQ[3^n+n], v=Append[v, n]; Print[v]], {n, 3, 19000}]
Select[Range[18500], PrimeQ[3^#+#]&] (* Harvey P. Dale, Jul 23 2013 *)
PROG
(PARI) is(n)=ispseudoprime(3^n+n) \\ Charles R Greathouse IV, May 22 2017
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Robert G. Wilson v, Nov 16 2000
EXTENSIONS
18248 from Farideh Firoozbakht, Aug 21 2003
STATUS
approved