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 A004502 #11 Oct 26 2023 08:31:46 %S A004502 13,14,12,16,17,15,10,11,9,22,23,21,25,26,24,19,20,18,4,5,3,7,8,6,1,2, %T A004502 0,40,41,39,43,44,42,37,38,36,49,50,48,52,53,51,46,47,45,31,32,30,34, %U A004502 35,33,28,29,27,67,68,66,70 %N A004502 Tersum n + 13. %F A004502 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 A004502 Cf. A004489. %K A004502 nonn %O A004502 0,1 %A A004502 _N. J. A. Sloane_