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 A006681 M3527 #26 Jul 08 2025 16:52:34
%S A006681 1,4,16,85,646,6664,86731,1354630,24607816,509000899,11802833296,
%T A006681 303129872500,8539567141561,261785254856584,8674697402099836,
%U A006681 308953306192527445,11768768651529113506,477433786852228938844
%N A006681 Number of binary phylogenetic trees with n labels.
%D A006681 Foulds, L. R.; Robinson, R. W. Enumeration of binary phylogenetic trees. Combinatorial mathematics, VIII (Geelong, 1980), pp. 187-202, Lecture Notes in Math., 884, Springer, Berlin-New York, 1981. Math. Rev. 83a:05071.
%D A006681 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A006681 L. R. Foulds and R. W. Robinson, <a href="/A006677/a006677.pdf">Enumeration of binary phylogenetic trees</a>, pp. 187-202, Lecture Notes in Math., 884, Springer, Berlin-New York, 1981. (Annotated scanned copy)
%H A006681 <a href="/index/Tra#trees">Index entries for sequences related to trees</a>
%F A006681 a(n) = Sum_{k=2..n} S(n, k) * A001147(k - 2) where S(n, m) are the Stirling numbers of the second kind. - _Sean A. Irvine_, Jun 16 2017
%F A006681 E.g.f.: ( -4 + 3*exp(x) + (3 - 2*exp(x))^(3/2) )/3. (see (3.1) in the L. R. Foulds and R. W. Robinson reference). - _Seiichi Manyama_, Sep 05 2024
%K A006681 nonn
%O A006681 2,2
%A A006681 _N. J. A. Sloane_
%E A006681 More terms and offset corrected by _Sean A. Irvine_, Jun 16 2017