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 A004501 #11 Oct 26 2023 08:31:42 %S A004501 12,13,14,15,16,17,9,10,11,21,22,23,24,25,26,18,19,20,3,4,5,6,7,8,0,1, %T A004501 2,39,40,41,42,43,44,36,37,38,48,49,50,51,52,53,45,46,47,30,31,32,33, %U A004501 34,35,27,28,29,66,67,68,69 %N A004501 Tersum n + 12. %F A004501 Tersum m + n: write m and n in base 3 and add mod 3 with no carries; e.g., 5 + 8 = "21" + "22" = "10" = 1. %Y A004501 Cf. A004489. %K A004501 nonn %O A004501 0,1 %A A004501 _N. J. A. Sloane_