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 A128461 #15 Jun 11 2021 08:40:31
%S A128461 1,2,4,10,21,25,27,32,60,88,106,120,146,264,828,965,1944,4822,12089,
%T A128461 14427,17354,42335,46395,58348,190632
%N A128461 Numbers k such that 21^k - 2 is a prime.
%C A128461 a(26) > 2*10^5. - _Robert Price_, Jul 14 2015
%t A128461 Do[ f = 21^n - 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n,1,1000} ]
%Y A128461 Cf. A084714 (smallest prime of the form (2n-1)^k - 2).
%Y A128461 Cf. A128472 (smallest prime of the form (2n-1)^k - 2 for k > (2n-1)).
%Y A128461 Cf. A014224, A109080, A090669, A128455, A128457, A128458, A128459, A128460.
%K A128461 nonn,hard,more
%O A128461 1,2
%A A128461 _Alexander Adamchuk_, Mar 14 2007
%E A128461 2 more terms from _Stefan Steinerberger_, May 05 2007
%E A128461 a(17)-a(25) from _Robert Price_, Jul 14 2015