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 A019553 #35 Dec 30 2023 23:43:27
%S A019553 15,210,315,420,525,630,735,840,945,1050,1155,1260,1365,1470,1575,
%T A019553 1680,1785,1890,1995,20100,21105,22110,23115,24120,25125,26130,27135,
%U A019553 28140,29145,30150,31155,32160,33165,34170,35175,36180,37185,38190,39195,40200
%N A019553 a(n) is the concatenation of n and 5n.
%C A019553 All terms are divisible by 15. - _Michel Marcus_, Sep 21 2015
%H A019553 Vincenzo Librandi, <a href="/A019553/b019553.txt">Table of n, a(n) for n = 1..1000</a>
%H A019553 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 A019553 a:=n->n*10^floor(log10(5*n)+1)+5*n: seq(a(n),n=1..50); # _Muniru A Asiru_, Jun 23 2018
%t A019553 n5n[n_]:=Module[{n5=5n},n*10^IntegerLength[n5]+n5]; Array[n5n,40] (* _Harvey P. Dale_, Apr 08 2012 *)
%t A019553 nxt[n_]:=Module[{idn=IntegerDigits[n], idn5=IntegerDigits[5n]}, FromDigits[Join[idn, idn5]]]; Array[nxt, 100] (* _Vincenzo Librandi_, Feb 04 2014 *)
%o A019553 (Magma) [Seqint(Intseq(5*n) cat Intseq(n)): n in [1..50]]; // _Vincenzo Librandi_, Feb 04 2014
%o A019553 (PARI) a(n) = eval(Str(n, 5*n)); \\ _Michel Marcus_, Sep 21 2015
%Y A019553 Cf. concatenation of n and k*n: A020338 (k=1), A019550 (k=2), A019551 (k=3), A019552 (k=4), this sequence (k=5), A009440 (k=6), A009441 (k=7), A009470 (k=8), A009474 (k=9).
%K A019553 nonn,base,easy,less
%O A019553 1,1
%A A019553 R. Muller