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 A097421 #7 Aug 07 2025 12:29:37
%S A097421 1,3,39,12207,2384185791,181898940354585647583,
%T A097421 2117582368135750847670806251699104905128479,
%U A097421 573971850987445072250359637315549647372395291392620860111695169081258427468128502368927
%N A097421 a(n) = (5^(2^n) - 1)/2^(n+2).
%t A097421 Table[(5^(2^n)-1)/2^(n+2),{n,0,10}] (* _Harvey P. Dale_, Aug 07 2025 *)
%Y A097421 Cf. A070632.
%K A097421 nonn
%O A097421 0,2
%A A097421 _Lekraj Beedassy_, Aug 21 2004