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 A228558 #10 Oct 01 2021 17:17:26 %S A228558 13,61,67,107,383,647,3571,37967 %N A228558 Numbers k such that (17^k + 4^k)/21 is prime. %C A228558 Conjecture: All terms are prime. %C A228558 a(9) > 10^5. %t A228558 Select[ Prime[ Range[1, 100000] ], PrimeQ[ (17^# + 4^#)/21 ]& ] %o A228558 (PARI) is(n)=ispseudoprime((17^n+4^n)/21) \\ _Charles R Greathouse IV_, May 22 2017 %Y A228558 Cf. A227049, A228225, A227046. %K A228558 hard,nonn,more %O A228558 1,1 %A A228558 _Robert Price_, Aug 25 2013