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 A019551 #30 Dec 30 2023 23:43:19
%S A019551 13,26,39,412,515,618,721,824,927,1030,1133,1236,1339,1442,1545,1648,
%T A019551 1751,1854,1957,2060,2163,2266,2369,2472,2575,2678,2781,2884,2987,
%U A019551 3090,3193,3296,3399,34102,35105,36108
%N A019551 a(n) is the concatenation of n and 3n.
%H A019551 Vincenzo Librandi, <a href="/A019551/b019551.txt">Table of n, a(n) for n = 1..1000</a>
%H A019551 Sylvester Smith, <a href="https://www.gallup.unm.edu/~smarandache/SYLSMITH.HTM">A Set of Conjectures on Smarandache Sequences</a>, Bulletin of Pure and Applied Sciences, (Bombay, India), Vol. 15 E (No. 1), 1996, pp. 101-107.
%p A019551 a:=n->n*10^floor(log10(3*n)+1)+3*n: seq(a(n),n=1..50); # _Muniru A Asiru_, Jun 23 2018
%t A019551 nxt[n_]:=Module[{idn=IntegerDigits[n], idn3=IntegerDigits[3n]}, FromDigits[Join[idn, idn3]]]; Array[nxt, 100] (* _Vincenzo Librandi_, Feb 04 2014 *)
%t A019551 Table[n*10^IntegerLength[3n]+3n,{n,40}] (* _Harvey P. Dale_, Apr 24 2022 *)
%o A019551 (Magma) [Seqint(Intseq(3*n) cat Intseq(n)): n in [1..50]]; // _Vincenzo Librandi_, Feb 04 2014
%Y A019551 Cf. concatenation of n and k*n: A020338 (k=1), A019550 (k=2), this sequence (k=3), A019552 (k=4), A019553 (k=5), A009440 (k=6), A009441 (k=7), A009470 (k=8), A009474 (k=9).
%K A019551 nonn,base,less,easy
%O A019551 1,1
%A A019551 R. Muller