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 A027881 #6 Dec 09 2019 22:27:30
%S A027881 1,1,2,4,9,20,51,121,321,826,2186,5789,16114,42449,118281,328425,
%T A027881 908933,2510553,7138324,19869748
%N A027881 a(i)=a(i-1)+a(j_1)*a(j_2) {where j_1+j_2=i-1, j_1 <= q j_2} + a(j_1)*a(j_2)*a(j_3) {where j_1+j_2+j_3=i-1, j_1 <= q j_2 <= q j_3} +...+ a(1)^{i-1}.
%C A027881 At one time it was thought that this might be related to the Hopf algebra structure of perturbative Quantum Field Theory; this later proved not to be the case.
%C A027881 Possibly erroneous version of A093637. - _Sean A. Irvine_, Dec 09 2019
%H A027881 Dirk Kreimer, <a href="http://arXiv.org/abs/q-alg/9707029">"On the Hopf algebra structure of perturbative quantum field theory"</a>
%H A027881 Dirk Kreimer, <a href="http://dipmza.physik.uni-mainz.de/~kreimer/homepage.html">Home page</a>
%K A027881 nonn
%O A027881 1,3
%A A027881 Dirk Kreimer (kreimer(AT)dipmza.physik.Uni-Mainz.DE)