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 A025646 #20 Jul 09 2025 12:49:16
%S A025646 0,1,0,2,1,0,3,2,1,0,4,3,2,1,0,5,4,3,2,1,0,6,5,4,3,2,1,0,7,6,5,4,3,2,
%T A025646 1,8,0,7,6,5,4,3,2,9,1,8,0,7,6,5,4,3,10,2,9,1,8,0,7,6,5,4,11,3,10,2,9,
%U A025646 1,8,0,7,6,5,12,4,11,3,10,2,9,1,8,0,7,6,13,5,12,4,11,3,10,2,9,1,8,0,7,14,6,13
%N A025646 Exponent of 4 (value of i) in n-th number of form 4^i*5^j.
%H A025646 Amiram Eldar, <a href="/A025646/b025646.txt">Table of n, a(n) for n = 1..10000</a>
%H A025646 <a href="/index/Se#sequences_which_agree_for_a_long_time">Index entries for sequences which agree for a long time but are different</a>.
%F A025646 a(n) = A235127(A025617(n)). - _Amiram Eldar_, Jul 09 2025
%t A025646 With[{nn=15},IntegerExponent[#,4]&/@Union[Flatten[Table[4^x 5^y,{x,0,nn},{y,0,nn}]]]] (* _Harvey P. Dale_, May 10 2019 *)
%t A025646 With[{max = 10^10}, IntegerExponent[Sort[Flatten[Table[4^i*5^j, {i, 0, Log[4, max]}, {j, 0, Log[5, max/4^i]}]]], 4]] (* _Amiram Eldar_, Jul 09 2025 *)
%Y A025646 Cf. A025617, A025650, A235127.
%Y A025646 Differs from A025661 at a(1881).
%K A025646 nonn
%O A025646 1,4
%A A025646 _David W. Wilson_