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 A009441 #17 Aug 02 2024 19:23:44
%S A009441 17,214,321,428,535,642,749,856,963,1070,1177,1284,1391,1498,15105,
%T A009441 16112,17119,18126,19133,20140,21147,22154,23161,24168,25175,26182,
%U A009441 27189,28196,29203,30210,31217,32224,33231,34238,35245,36252,37259,38266,39273,40280
%N A009441 a(n) is the concatenation of n and 7n.
%H A009441 Vincenzo Librandi, <a href="/A009441/b009441.txt">Table of n, a(n) for n = 1..1000</a>
%t A009441 nxt[n_]:=Module[{idn=IntegerDigits[n], idn7=IntegerDigits[7n]}, FromDigits[Join[idn, idn7]]]; Array[nxt, 100] (* _Vincenzo Librandi_, Feb 04 2014 *)
%t A009441 Table[n*10^IntegerLength[7n]+7n,{n,40}] (* _Harvey P. Dale_, Aug 02 2024 *)
%o A009441 (Magma) [Seqint(Intseq(7*n) cat Intseq(n)): n in [1..50]]; // _Vincenzo Librandi_, Feb 04 2014
%Y A009441 Cf. concatenation of n and k*n: A020338 (k=1), A019550 (k=2), A019551 (k=3), A019552 (k=4), A019553 (k=5), A009440 (k=6), this sequence (k=7), A009470 (k=8), A009474 (k=9).
%K A009441 nonn,base,less,easy
%O A009441 1,1
%A A009441 _David W. Wilson_