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 A129880 #10 May 04 2018 05:52:09
%S A129880 1,1,4,10,32,101,334,1129,3890,13618,48275,172995,625574,2279997,
%T A129880 8366553,30885640,114620926,427383918,1600323161,6015226752,
%U A129880 22687991095,85843131701,325734335544,1239274254094,4726359853214,18065978123065
%N A129880 Sequence d_n arising in enumeration of arrays of directed blocks (see Quaintance reference for precise definition).
%H A129880 Jocelyn Quaintance, <a href="https://doi.org/10.1016/j.disc.2006.09.041">Combinatoric Enumeration of Two-Dimensional Proper Arrays</a>, Discrete Math., 307 (2007), 1844-1864.
%F A129880 See Quaintance reference for generating functions that produce A129872-A129886.
%F A129880 G.f.: D(t) satisfies (t^3 + t^2)*D^3 + (t^3 - t^2 - 2*t)*D^2 + (-t^2 - t + 1)*D - t = 0. _Michel Marcus_, Mar 29 2013
%o A129880 (PARI) listdn(m) = { my(D = t*O(t)); for (n= 1, m, D = (t - (t^3 + t^2)*D^3 - (t^3 - t^2 - 2*t)*D^2)/(1 - t - t^2);); return(vector(m, i , polcoeff(D, i, t)));} \\ _Michel Marcus_, Mar 29 2013
%K A129880 nonn
%O A129880 1,3
%A A129880 _N. J. A. Sloane_, May 26 2007