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 A055966 #5 Sep 15 2022 21:27:34 %S A055966 0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,21,42,63,84, %T A055966 105,126,147,168,189,210,231,252,273,294,315,336,357,378,399,420,42, %U A055966 63,84,105,126,147,168,189,210,231,252,273,294,315,336,357,378,399,420 %N A055966 n + reversal of base 20 digits of n (written in base 10). %C A055966 If n has an even number of base 20 digits then a(n) is a multiple of 21. %F A055966 a(n) = n + A056963(n). %Y A055966 Cf. A056964. %K A055966 base,nonn %O A055966 0,2 %A A055966 _Henry Bottomley_, Jul 18 2000