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 A138049 #25 Jun 07 2021 05:11:04
%S A138049 0,1,2,4,7,24,40,112,310,1026,1286,36566,43717,53753
%N A138049 Numbers k such that 21^k + 2 is prime.
%C A138049 No further terms < 100000. - _Ray Chandler_, Aug 11 2011
%C A138049 a(15) > 2*10^5. - _Robert Price_, Jul 14 2015
%H A138049 Henri & Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=21%5En%2B2&action=Search">PRP Records</a>.
%t A138049 Do[ f = 21^n + 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n, 1, 310} ]
%o A138049 (PARI) is(n)=ispseudoprime(21^n+2) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y A138049 Cf. A051783 (k such that 3^k + 2 is prime).
%Y A138049 Cf. A087885 (k such that 5^k + 2 is prime).
%Y A138049 Cf. A090649, A109076, A113480, A138048, A138050, A138051, A087886, A113481.
%K A138049 nonn,more
%O A138049 1,3
%A A138049 _Alexander Adamchuk_, Mar 02 2008
%E A138049 1026 from Herman Jamke (hermanjamke(AT)fastmail.fm), May 01 2008
%E A138049 1286 and 36566 from _Ray Chandler_, Jul 31 2011
%E A138049 43717 from _Ray Chandler_, Aug 01 2011
%E A138049 53753 from _Ray Chandler_, Aug 02 2011