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 A059249 #6 Nov 09 2021 18:22:37 %S A059249 1,0,5,7,6,2,4,3,17,19,18,23,25,24,20,22,21,8,10,9,14,16,15,11,13,12, %T A059249 53,55,54,59,61,60,56,58,57,71,73,72,77,79,78,74,76,75,62,64,63,68,70, %U A059249 69,65,67,66,26,28,27,32,34,33,29,31,30,44,46,45,50,52,51,47,49,48,35 %N A059249 Tersum n + (n-1); write n and n-1 in base 3 and add mod 3 with no carries. %H A059249 Alois P. Heinz, <a href="/A059249/b059249.txt">Table of n, a(n) for n = 1..19682</a> %e A059249 a(21)=14 since 21 and 20 are written in base 3 as 210 and 202 and so their tersum is 112 in base 3, i.e. 9+3+2=14. %Y A059249 Cf. A004482, A004488, A004489, A038712. %K A059249 nonn,look %O A059249 1,3 %A A059249 _Henry Bottomley_, Jan 22 2001