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 A325521 #7 May 06 2019 21:23:21
%S A325521 2,1,3,1,2,3,1,1,2,2,2,4,2,1,3,3,1,2,2,1,3,1,2,1,2,1,1,4,3,3,1,1,1,3,
%T A325521 1,1,4,2,4,1,2,3,1,1,2,2,2,5,1,1,6,1,2,2,1,3,1,1,1,3,1,1,2,2,6,2,1,3,
%U A325521 3,1,2,1,1,4,1,2,1,2,1,1,4,2,1,3,1,1
%N A325521 Difference sequence of A325428.
%H A325521 Clark Kimberling, <a href="/A325521/b325521.txt">Table of n, a(n) for n = 1..10000</a>
%e A325521 See A325417 for a guide to related sequences. Conjecture: every term is in {1,2,3,4,5,6,7,8,9}.
%t A325521 a = {1}; Do[AppendTo[a, NestWhile[# + 1 &, Last[a] + 1, Apply[Or,
%t A325521 Map[MemberQ[a, #] &, Select[Flatten[{(# - 1)/2,
%t A325521 If[Mod[#, 3] == 0, (2 #)/3, 0] + If[Mod[#, 3] == 1, 1/3 (1 + 2 #), 0]}],
%t A325521 IntegerQ || # == 0]]] &]], {200}]; a ; (* A325427 *)
%t A325521 c = Complement[Range[Last[a]], a] ; (* A325428 *)
%t A325521 Differences[a] (* A325520 *)
%t A325521 Differences[c] (* A325521 *)
%t A325521 (* _Peter J. C. Moses_, Apr 23 2019 *)
%Y A325521 Cf. A325417, A325428, A325520.
%K A325521 nonn,easy
%O A325521 1,1
%A A325521 _Clark Kimberling_, May 06 2019