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%I #11 May 05 2021 18:16:09
%S 0,1,3,5,11,15,21,87,99,281,497,2919,6849,7365,8483,49317,58611
%N Numbers n such that 9^n + 4 is prime.
%C Contribution from _Bruno Berselli_, Oct 04 2012: (Start)
%C Contains exactly the halved even terms of A058958.
%C Naturally, apart from the first term, these numbers are odd since (10-1)^(2n)+4 is divisible by 5. (End)
%C a(18) > 10^5. - _Tyler NeSmith_, May 05 2021
%t Select[Range[0, 3000], PrimeQ[9^# + 4] &]
%o (PARI) is(n)=ispseudoprime(9^n+4) \\ _Charles R Greathouse IV_, Feb 20 2017
%Y Cf. A058958, A090649.
%K nonn,hard,more
%O 1,3
%A _Vincenzo Librandi_, Oct 04 2012
%E a(13)-a(14) from _Bruno Berselli_, Oct 04 2012
%E a(15)-a(17) from _Tyler NeSmith_, May 05 2021
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