This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A217384 #15 Oct 31 2024 11:23:25 %S A217384 0,1,3,5,11,15,21,87,99,281,497,2919,6849,7365,8483,49317,58611 %N A217384 Numbers k such that 9^k + 4 is prime. %C A217384 Contribution from _Bruno Berselli_, Oct 04 2012: (Start) %C A217384 Contains exactly the halved even terms of A058958. %C A217384 Naturally, apart from the first term, these numbers are odd since (10-1)^(2n)+4 is divisible by 5. (End) %C A217384 a(18) > 10^5. - _Tyler NeSmith_, May 05 2021 %t A217384 Select[Range[0, 3000], PrimeQ[9^# + 4] &] %o A217384 (PARI) is(n)=ispseudoprime(9^n+4) \\ _Charles R Greathouse IV_, Feb 20 2017 %Y A217384 Cf. A058958, A090649. %K A217384 nonn,hard,more %O A217384 1,3 %A A217384 _Vincenzo Librandi_, Oct 04 2012 %E A217384 a(13)-a(14) from _Bruno Berselli_, Oct 04 2012 %E A217384 a(15)-a(17) from _Tyler NeSmith_, May 05 2021