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 A137309 #6 Dec 27 2023 00:53:26
%S A137309 1,35,791,3579,13579,135791,3579135,79135791,357913579,1357913579,
%T A137309 13579135791,357913579135,7913579135791,35791357913579,
%U A137309 135791357913579,1357913579135791,35791357913579135,791357913579135791,3579135791357913579
%N A137309 Let T = {1,3,5,7,9,1,3,5,7,9,1,3,5,7,9, ... }; a(n) is n-th concatenation of n numbers from T.
%Y A137309 Cf. A141518.
%K A137309 easy,nonn,base
%O A137309 1,2
%A A137309 _Ctibor O. Zizka_, Apr 05 2008
%E A137309 Edited by _Charles R Greathouse IV_, Apr 25 2010