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 A235497 #30 Sep 08 2022 08:46:06
%S A235497 21,42,63,84,105,126,147,168,189,2010,2211,2412,2613,2814,3015,3216,
%T A235497 3417,3618,3819,4020,4221,4422,4623,4824,5025,5226,5427,5628,5829,
%U A235497 6030,6231,6432,6633,6834,7035,7236,7437,7638,7839,8040,8241,8442,8643,8844,9045
%N A235497 2n concatenated with n.
%C A235497 All terms are divisible by 3. - _Michel Marcus_, Sep 21 2015
%H A235497 Vincenzo Librandi, <a href="/A235497/b235497.txt">Table of n, a(n) for n = 1..1000</a>
%F A235497 a(n) = 2*n*10^floor(log_10(n) + 1) + n.
%t A235497 nxt[n_] := Module[{idn = IntegerDigits[n], idn2 = IntegerDigits[2 n]}, FromDigits[Join[idn2, idn]]]; Array[nxt, 50] (* Librandi *)
%t A235497 Table[FromDigits[Join[IntegerDigits[2n], IntegerDigits[n]]], {n, 40}] (* _Alonso del Arte_, Sep 13 2014 *)
%o A235497 (Magma) [Seqint(Intseq(n) cat Intseq(2*n)): n in [1..50]];
%o A235497 (Maxima) makelist(2*n*10^floor(log(n)/log(10) + 1) + n,n,50); /* _Bruno Berselli_, Feb 06 2014, using the closed form added from _Alois P. Heinz_ on Feb 05 2014 */
%o A235497 (PARI) a(n) = eval(Str(2*n, n)); \\ _Michel Marcus_, Sep 21 2015
%Y A235497 Cf. A019550.
%K A235497 nonn,base,easy
%O A235497 1,1
%A A235497 _Vincenzo Librandi_, Feb 04 2014