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 A007943 #25 Dec 23 2023 14:23:27
%S A007943 12,1342,135642,13578642,13579108642,135791112108642,
%T A007943 1357911131412108642,13579111315161412108642,
%U A007943 135791113151718161412108642,1357911131517192018161412108642
%N A007943 Concatenation of sequence (1,3,..,2n-1,2n,2n-2,..,2).
%D A007943 M. Le, Perfect Powers in the Smarandache Permutation Sequence, Smarandache Notions Journal, Vol. 10, No. 1-2-3, 1999, 148-149.
%H A007943 Harvey P. Dale, <a href="/A007943/b007943.txt">Table of n, a(n) for n = 1..184</a>
%H A007943 F. Smarandache, <a href="http://www.gallup.unm.edu/~smarandache/OPNS.pdf">Only Problems, Not Solutions!</a>
%t A007943 Table[FromDigits[Join[Flatten[IntegerDigits/@Range[1,2n+1,2]],Flatten[ IntegerDigits/@ Range[2n+2,2,-2]]]],{n,0,10}] (* _Harvey P. Dale_, Jul 30 2021 *)
%Y A007943 Cf. A007942.
%K A007943 nonn,base
%O A007943 1,1
%A A007943 R. Muller