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 A135020 #10 Aug 17 2025 17:01:32
%S A135020 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,1,11,11,12,21,13,31,14,41,15,
%T A135020 51,16,61,17,71,18,81,19,91,20,2,21,12,22,22,23,32,24,42,25,52,26,62,
%U A135020 27,72,28,82,29,92,30,3,31,13,32,23,33,33,34,43,35,53,36,63,37,73,38,83,39,93
%N A135020 Each natural number is followed by its reversal.
%H A135020 Alois P. Heinz, <a href="/A135020/b135020.txt">Table of n, a(n) for n = 1..10000</a>
%F A135020 a(n) = (n+1)/2 for n = odd, a(n) = A004086(n/2) for n = even.
%t A135020 {#,IntegerReverse[#]}&/@Range[40]//Flatten (* _Harvey P. Dale_, Aug 17 2025 *)
%Y A135020 Cf. A004086.
%K A135020 easy,nonn,base,look
%O A135020 1,3
%A A135020 _Rolf Pleisch_, Feb 10 2008