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 A009440 #19 Sep 08 2022 08:44:37
%S A009440 16,212,318,424,530,636,742,848,954,1060,1166,1272,1378,1484,1590,
%T A009440 1696,17102,18108,19114,20120,21126,22132,23138,24144,25150,26156,
%U A009440 27162,28168,29174,30180,31186,32192,33198,34204,35210,36216,37222,38228,39234,40240
%N A009440 a(n) is the concatenation of n and 6n.
%H A009440 Vincenzo Librandi, <a href="/A009440/b009440.txt">Table of n, a(n) for n = 1..1000</a>
%t A009440 nxt[n_]:=Module[{idn=IntegerDigits[n], idn6=IntegerDigits[6n]}, FromDigits[Join[idn, idn6]]]; Array[nxt, 100] (* _Vincenzo Librandi_, Feb 04 2014 *)
%t A009440 Table[n*10^IntegerLength[6n]+6n,{n,40}] (* _Harvey P. Dale_, Jul 21 2020 *)
%o A009440 (Magma) [Seqint(Intseq(6*n) cat Intseq(n)): n in [1..50]]; // _Vincenzo Librandi_, Feb 04 2014
%Y A009440 Cf. concatenation of n and k*n: A020338 (k=1), A019550 (k=2), A019551 (k=3), A019552 (k=4), A019553 (k=5), this sequence (k=6), A009441 (k=7), A009470 (k=8), A009474 (k=9).
%K A009440 nonn,base,less,easy
%O A009440 1,1
%A A009440 _David W. Wilson_