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 A270647 #11 Jun 21 2025 15:24:38
%S A270647 2,2,1,2,1,1,2,2,1,2,2,1,2,1,1,2,2,1,2,2,1,1,2,1,1,2,1,2,2,1,1,2,1,1,
%T A270647 2,2,1,2,1,1,2,1,2,2,1,2,2,1,1,2,1,2,2,1,2,1,1,2,2,1,2,2,1,2,1,1,2,1,
%U A270647 1,2,2,1,2,2,1,1,2,1,2,2,1,2,2,1,1,2
%N A270647 The sequence a of 1's and 2's starting with (2,2,1,2) such that a(n) is the length of the (n+3)rd run of a.
%C A270647 See A270641 for a guide to related sequences.
%H A270647 Clark Kimberling, <a href="/A270647/b270647.txt">Table of n, a(n) for n = 1..10000</a>
%F A270647 Conjecture: a(n) = A270643(n+1). - _R. J. Mathar_, Jun 21 2025
%e A270647 a(1) = 2, so the 4th run has length 2, so a(5) must be 1 and a(6) = 1.
%e A270647 a(2) = 2, so the 5th run has length 2, so a(7) = 2 and a(8) = 2.
%e A270647 a(3) = 1, so the 6th run has length 1, so a(9) = 1 and a(10) = 2.
%e A270647 Globally, the runlength sequence of a is 2,1,1,2,2,1,2,1,1,2,2,1,2,2,1,2,1,1,2,..., and deleting the first 3 terms leaves a = A270647.
%t A270647 a = {2,2,1,2}; Do[a = Join[a, ConstantArray[If[Last[a] == 1, 2, 1], {a[[n]]}]], {n, 200}]; a (* _Peter J. C. Moses_, Apr 01 2016 *)
%Y A270647 Cf. A000002, A006928, A022300, A270641.
%K A270647 nonn,easy
%O A270647 1,1
%A A270647 _Clark Kimberling_, Apr 07 2016