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 A007855 #19 Apr 05 2019 10:06:14
%S A007855 1,3,13,63,326,1769,9964,57843,344203,2090470,12912988,80899801,
%T A007855 512896540,3284651548,21217493460,138080484819,904454380446,
%U A007855 5958186674879,39448465279220,262359379484522,1751912981641794,11741044418866476
%N A007855 Infima closed sets in rooted plane trees on n nodes.
%H A007855 M. Klazar, <a href="http://dx.doi.org/10.1006/eujc.1995.0095">Twelve countings with rooted plane trees</a>, European Journal of Combinatorics 18 (1997), 195-210; Addendum, 18 (1997), 739-740.
%H A007855 <a href="/index/Ro#rooted">Index entries for sequences related to rooted trees</a>
%F A007855 G.f.: (1/4) * (1 + sqrt(1-4*x)) * (1 - sqrt(3 - 2 / sqrt(1-4*x))) [from Klazar]. - _Sean A. Irvine_, Feb 06 2018
%o A007855 (PARI) my(x='x+O('x^30)); Vec((1/4) * (1+sqrt(1-4*x)) * (1-sqrt(3-2/sqrt(1-4*x)))) \\ _Michel Marcus_, Apr 05 2019
%K A007855 nonn
%O A007855 1,2
%A A007855 _Martin Klazar_
%E A007855 More terms from _Sean A. Irvine_, Feb 06 2018