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 A043883 #13 Aug 25 2021 04:53:10
%S A043883 6,9,22,27,39,41,54,57,70,73,86,91,111,118,121,134,137,159,167,169,
%T A043883 182,185,198,201,214,219,231,233,246,249,262,265,278,283,295,297,310,
%U A043883 313,326,329,342,347,367,374,377,447,454,457,470
%N A043883 Numbers k such that 1 and 2 occur juxtaposed in the base-4 representation of k but not of k+1.
%t A043883 SequencePosition[Table[If[SequenceCount[IntegerDigits[n,4],{1,2}]>0 || SequenceCount[IntegerDigits[n,4],{2,1}]>0,1,0],{n,500}],{1,0}][[All,1]] (* _Harvey P. Dale_, Aug 05 2021 *)
%Y A043883 Cf. A007090.
%K A043883 nonn,base
%O A043883 1,1
%A A043883 _Clark Kimberling_