OFFSET
1,3
COMMENTS
Contribution from Bruno Berselli, Oct 04 2012: (Start)
Contains exactly the halved even terms of A058958.
Naturally, apart from the first term, these numbers are odd since (10-1)^(2n)+4 is divisible by 5. (End)
a(18) > 10^5. - Tyler NeSmith, May 05 2021
MATHEMATICA
Select[Range[0, 3000], PrimeQ[9^# + 4] &]
PROG
(PARI) is(n)=ispseudoprime(9^n+4) \\ Charles R Greathouse IV, Feb 20 2017
CROSSREFS
KEYWORD
nonn,hard,more,changed
AUTHOR
Vincenzo Librandi, Oct 04 2012
EXTENSIONS
a(13)-a(14) from Bruno Berselli, Oct 04 2012
a(15)-a(17) from Tyler NeSmith, May 05 2021
STATUS
approved