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 A053333 #7 Jul 14 2023 13:21:05
%S A053333 4,44,944,4944,94944,994944,4994944,94994944,494994944,9494994944,
%T A053333 49494994944,449494994944,9449494994944,99449494994944,
%U A053333 499449494994944,9499449494994944,49499449494994944,949499449494994944
%N A053333 a(n) contains n digits (either '4' or '9') and is divisible by 2^n.
%H A053333 Ray Chandler, <a href="/A053333/b053333.txt">Table of n, a(n) for n = 1..1000</a>
%F A053333 a(n)=a(n-1)+10^(n-1)*(4+5*[a(n-1)/2^(n-1) mod 2]) i.e. a(n) ends with a(n-1); if (n-1)-th term is divisible by 2^n then n-th term begins with a 4, if not then n-th term begins with a 9.
%t A053333 nxt[{n_,a_}]:=Module[{fd=If[Mod[a,2^(n+1)]==0,4,9]},{n+1,fd 10^IntegerLength[a]+a}]; NestList[ nxt,{1,4},20][[;;,2]] (* _Harvey P. Dale_, Jul 14 2023 *)
%Y A053333 Cf. A023405, A050621, A050622, A035014.
%K A053333 base,nonn
%O A053333 1,1
%A A053333 _Henry Bottomley_, Mar 06 2000