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 A124621 #22 May 04 2021 20:04:39 %S A124621 0,2,6,10,102,494,794,1326,5242,5446,24602,87606,188558 %N A124621 Numbers k such that 5^k + 4 is prime. %C A124621 a(13) > 10^5. - _Robert Price_, Feb 03 2014 %C A124621 a(14) > 2*10^5. - _Tyler NeSmith_, May 04 2021 %H A124621 F. Firoozbakht, M. F. Hasler, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL13/Hasler/hasler2.html">Variations on Euclid's formula for Perfect Numbers</a>, JIS 13 (2010) #10.3.1 %e A124621 5^2 + 4 = 29 is prime, so 2 is a term. %t A124621 Select[Range[0, 2000], PrimeQ[5^# + 4] &] (* _Vincenzo Librandi_, Oct 01 2012 *) %o A124621 (PARI) is(n)=ispseudoprime(5^n+4) \\ _Charles R Greathouse IV_, Feb 17 2017 %Y A124621 Cf. A059613. %K A124621 nonn %O A124621 1,2 %A A124621 _Artur Jasinski_, Dec 21 2006 %E A124621 Added the first term 0 by _Vincenzo Librandi_, Oct 01 2012 %E A124621 a(9) - a(10) from _Bruno Berselli_, Aug 12 2013 %E A124621 a(11)-a(12) from _Robert Price_, Feb 03 2014 %E A124621 a(13) from _Tyler NeSmith_, May 04 2021