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 A043884 #13 Aug 25 2021 04:53:14
%S A043884 7,13,23,31,39,45,55,61,71,77,87,95,103,109,127,135,141,151,159,167,
%T A043884 173,183,189,199,205,223,231,237,247,253,263,269,279,287,295,301,311,
%U A043884 317,327,333,343,351,359,365,383,391,397,407,415
%N A043884 Numbers k such that 1 and 3 occur juxtaposed in the base-4 representation of k but not of k+1.
%t A043884 SequencePosition[Table[If[SequenceCount[IntegerDigits[n,4],{1,3}]>0 || SequenceCount[IntegerDigits[n,4],{3,1}]>0,1,0],{n,500}],{1,0}][[All,1]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, May 13 2020 *)
%Y A043884 Cf. A007090.
%K A043884 nonn,base
%O A043884 1,1
%A A043884 _Clark Kimberling_