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 A004073 #15 Mar 31 2017 22:50:57 %S A004073 1,12,1123,11121234,1111211212312345,11111211121121231121231234123456, %T A004073 1111112111121112112123111211212311212312341121231234123451234567 %N A004073 Start with a(1)=1; to get a(n) replace each i in a(n-1) with 12...i, then append n. %C A004073 Digits sum to A000225. %H A004073 T. Kubo and R. Vakil, <a href="http://dx.doi.org/10.1016/0012-365X(94)00303-Z">On Conway's recursive sequence</a>, Discr. Math. 152 (1996), 225-252. %K A004073 nonn,base %O A004073 1,2 %A A004073 _N. J. A. Sloane_ %E A004073 The next term is too large to include.