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 A025666 #12 Aug 12 2019 02:23:04
%S A025666 0,0,1,0,1,2,0,1,2,0,3,1,2,0,3,1,4,2,0,3,1,4,2,0,5,3,1,4,2,0,5,3,1,6,
%T A025666 4,2,0,5,3,1,6,4,2,7,0,5,3,1,6,4,2,7,0,5,3,8,1,6,4,2,7,0,5,3,8,1,6,4,
%U A025666 9,2,7,0,5,3,8,1,6,4,9,2,7,0,5,10,3,8,1,6,4,9,2,7,0,5,10,3,8,1,6,11,4,9,2,7,0,5
%N A025666 Exponent of 7 (value of j) in n-th number of form 4^i*7^j.
%H A025666 Robert Israel, <a href="/A025666/b025666.txt">Table of n, a(n) for n = 1..10000</a>
%F A025666 4^A025648(n)*7^a(n) = A025619(n). - _Robert Israel_, Aug 11 2019
%p A025666 N:= 10^10: # to consider all terms with 4^i*7^j<=N
%p A025666 S:= {seq(seq(4^i*7^j, j=0..floor(log[7](N/4^i))),i=0..floor(log[4](N)))}:
%p A025666 map(padic:-ordp, sort(convert(S,list)),7); # _Robert Israel_, Aug 11 2019
%Y A025666 Cf. A025619, A025648.
%K A025666 nonn
%O A025666 1,6
%A A025666 _David W. Wilson_