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 A000643 #21 Jan 05 2024 19:14:02
%S A000643 0,0,1,2,4,8,24,280,16777496,
%T A000643 1942668892225729070919461906823518906642406839052139521251812409738904285205225275672
%N A000643 a(n) = a(n-1) + 2^a(n-2).
%C A000643 The next term is too large to include.
%C A000643 The next term has 5050530 digits. - _Harvey P. Dale_, Jan 05 2024
%D A000643 D. E. Knuth, personal communication.
%t A000643 Off[General::ovfl];a={1,0};Flatten[Prepend[Table[s=Plus@@a;a=2^(RotateLeft[a]);a[[ -1]]=s,{n,8}],Table[0,{m,Length[a]-1}]]] (* _Vladimir Joseph Stephan Orlovsky_, Nov 18 2009 *)
%t A000643 nxt[{a_,b_}]:={b,b+2^a}; NestList[nxt,{0,0},9][[;;,1]] (* _Harvey P. Dale_, Jan 05 2024 *)
%Y A000643 Cf. A000659.
%K A000643 nonn
%O A000643 0,4
%A A000643 _N. J. A. Sloane_
%E A000643 1 more term from _Vladeta Jovovic_, Mar 19 2000