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 A129137 #15 Nov 15 2019 21:31:31
%S A129137 0,0,1,5,37,366,4553,68408,1206405,24447440,560041201,14315792256,
%T A129137 404057805989,12482986261760,419042630871225,15189786100468736,
%U A129137 591374264243364037,24612549706061862912,1090556290466098198625
%N A129137 Number of trees on [n], rooted at 1, in which 2 is a descendant of 3.
%H A129137 Washington G. Bomfim, <a href="/A129137/b129137.txt">Table of n, a(n) for n = 1..50</a>
%H A129137 H. Bergeron, E. M. F. Curado, J. P. Gazeau and L. M. C. S. Rodrigues, <a href="http://arxiv.org/abs/1309.6910">A note about combinatorial sequences and Incomplete Gamma function</a>, arXiv preprint arXiv: 1309.6910, 2013
%F A129137 The following formula counts these trees by the length r of the path from 1 to 3: Sum_{r=1..n-2} (n-3)!*n^(n-2-r)/(n-2-r)!.
%e A129137 a(4)=5 counts {1->3->2, 1->4}, {1->3->2, 3->4}, {1->3->2->4}, {1->3->4->2}, {1->4->3->2}.
%t A129137 Table[Exp[n]*Gamma[n-2, n] // Round, {n, 1, 50}] (* _Jean-François Alcover_, Jan 15 2014 *)
%Y A129137 Cf. A057500 = binomial(n-1, 2)*a(n).
%K A129137 nonn
%O A129137 1,4
%A A129137 _David Callan_, Mar 30 2007