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 A270645 #9 Oct 09 2020 10:42:59
%S A270645 2,1,1,1,2,2,1,2,1,2,2,1,1,2,1,1,2,1,1,2,2,1,2,1,1,2,1,2,2,1,2,1,1,2,
%T A270645 2,1,2,2,1,2,1,1,2,1,1,2,2,1,2,2,1,2,1,1,2,2,1,2,2,1,1,2,1,1,2,1,2,2,
%U A270645 1,2,1,1,2,2,1,2,2,1,1,2,1,1,2,1,2,2
%N A270645 The sequence a of 1's and 2's starting with (2,1,1,1) such that a(n) is the length of the (n+2)nd run of a.
%C A270645 See A270641 for a guide to related sequences.
%H A270645 Clark Kimberling, <a href="/A270645/b270645.txt">Table of n, a(n) for n = 1..10000</a>
%e A270645 a(1) = 1, so the 3rd run has length 2, so a(5) must be 2 and a(6) = 2.
%e A270645 a(2) = 2, so the 4th run has length 1, so a(7) = 1 and a(8) = 2.
%e A270645 a(3) = 1, so the 5th run has length 1, so a(9) = 1 and a(10) = 2.
%e A270645 Globally, the runlength sequence is 1,3,2,1,1,1,2,2,1,2,1,2,2,1,1,..., and deleting the first 2 terms leaves the same sequence.
%t A270645 a = {2,1,1,1}; Do[a = Join[a, ConstantArray[If[Last[a] == 1, 2, 1], {a[[n]]}]], {n, 200}]; a (* _Peter J. C. Moses_, Apr 01 2016 *)
%Y A270645 Cf. A000002, A006928, A022300, A270641.
%K A270645 nonn,easy
%O A270645 1,1
%A A270645 _Clark Kimberling_, Apr 06 2016