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 A043881 #11 Aug 25 2021 04:53:01 %S A043881 8,18,24,35,40,50,56,66,75,82,88,99,104,114,120,143,146,152,163,168, %T A043881 178,184,194,203,210,216,227,232,242,248,258,267,274,280,303,306,312, %U A043881 322,331,338,344,355,360,370,376,399,402,408,419 %N A043881 Numbers k such that 0 and 2 occur juxtaposed in the base-4 representation of k but not of k+1. %Y A043881 Cf. A007090. %K A043881 nonn,base %O A043881 1,1 %A A043881 _Clark Kimberling_