OFFSET
1,2
COMMENTS
The primes are in A172996.
Naturally these numbers are odd since 3^(2*k) + 7 = (8+1)^k + 7 == 0 (mod 8). - Bruno Berselli, Oct 08 2012
For n <= 9, (3^a(n) + 7)/2 has been proved prime using PARI's ECPP; a(10) corresponds to a BPSW PRP. a(11) > 10^5. - Lucas A. Brown, Feb 10 2021
LINKS
Lucas A. Brown, A172995.py
MATHEMATICA
Select[Range[5000], PrimeQ[(3^# + 7) / 2 ]&] (* Vincenzo Librandi, Oct 05 2012 *)
PROG
(PARI) is(n)=ispseudoprime((3^n+7)/2) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Vincenzo Librandi, Feb 07 2010
EXTENSIONS
a(7)-(9) from Vincenzo Librandi, Oct 05 2012
a(10) from Lucas A. Brown, Feb 10 2021
a(11) from Michael S. Branicky, Sep 07 2024
STATUS
approved