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 A255841 #35 Oct 21 2015 11:25:34 %S A255841 1,2,6,25,137,945,7927,78731,906705,11908357,175978520,2893866042, %T A255841 52467157456,1040596612520,22425725219277,522102436965475, %U A255841 13064892459014192,349829488635512316 %N A255841 The number of unordered binary trees that contain n distinct subtrees. %C A255841 Each node has no more than two children and the relative order of children is unimportant. See A254789 for the ordered case. %C A255841 The set of subtrees includes the full tree as well as the union of those in the principal subtrees. %C A255841 A lower bound for the sequence is (n-1)!, which counts trees where one principal subtree is contained in the other (including trees with only one principal subtree). %C A255841 a(n) is also the number of unordered full binary trees with n+1 subtrees. %H A255841 Andrew Szymczak, <a href="/A255841/b255841.txt">Table of n, a(n) for n = 1..80</a> %H A255841 Philippe Flajolet et al., <a href="http://dx.doi.org/10.1007/BFb0032034">Analytic variations on the common subexpression problem</a>, Automata, Languages and Programming, Lecture Notes in Computer Science, Volume 443, 1990, pp 220-234. %H A255841 Project Euler.net, <a href="http://forum.projecteuler.net/viewtopic.php?f=16&t=3987">Counting Subtrees</a>, Project Euler Forums, (2015). %H A255841 Andrew Szymczak et al., <a href="http://math.stackexchange.com/questions/1132147/egf-of-rooted-minimal-directed-acylic-graph">Distinct subtrees in binary trees</a>, Math StackExchange, (2015). %Y A255841 Cf. A254789. %K A255841 nonn,hard %O A255841 1,2 %A A255841 _Andrew Szymczak_, Mar 07 2015