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 A053315 #8 Oct 27 2019 17:30:10 %S A053315 4,44,544,4544,44544,444544,4444544,54444544,454444544,5454444544, %T A053315 45454444544,545454444544,5545454444544,55545454444544, %U A053315 555545454444544,4555545454444544,44555545454444544,544555545454444544 %N A053315 a(n) contains n digits (either '4' or '5') and is divisible by 2^n. %H A053315 Robert Israel, <a href="/A053315/b053315.txt">Table of n, a(n) for n = 1..999</a> %F A053315 a(n) = a(n-1) + 10^(n-1)*(4 + (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 5. %p A053315 A[1]:= 4: %p A053315 for n from 2 to 100 do %p A053315 if A[n-1] mod 2^n = 0 then A[n]:= A[n-1]+4*10^(n-1) %p A053315 else A[n]:= A[n-1]+5*10^(n-1) %p A053315 fi %p A053315 od: %p A053315 seq(A[i],i=1..100); # _Robert Israel_, Oct 27 2019 %Y A053315 Cf. A023403, A050621, A050622, A035014. %K A053315 base,nonn %O A053315 1,1 %A A053315 _Henry Bottomley_, Mar 06 2000 %E A053315 Formula corrected by _Robert Israel_, Oct 27 2019