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 A220817 #15 Sep 07 2019 15:11:45
%S A220817 4,10,40,215,1260,8010,53240,366680,2590420,18674660,136809240,
%T A220817 1015603015,7622900360,57753364510,441081947840,3392237023425,
%U A220817 26248421645200,204202630151860,1596256637347240,12531697773337965,98763817012822460,781102051887068160
%N A220817 Number of rooted binary MUL-trees with n leaves on the label set [4].
%H A220817 Andrew Howroyd, <a href="/A220817/b220817.txt">Table of n, a(n) for n = 1..200</a>
%H A220817 V. P. Johnson, <a href="http://people.math.sc.edu/czabarka/Theses/JohnsonThesis.pdf">Enumeration Results on Leaf Labeled Trees</a>, Ph. D. Dissertation, Univ. Southern Calif., 2012.
%t A220817 a[n_] := a[n] = If[n < 2, 4 n, If[OddQ[n], 0, (# (1 - #)/2 &)[a[n/2]]] + Sum[a[i] a[n - i], {i, 1, n/2}]];
%t A220817 Array[a, 25] (* _Jean-François Alcover_, Sep 02 2019, after _Alois P. Heinz_ in A319539 *)
%Y A220817 Column k=4 of A319539.
%K A220817 nonn
%O A220817 1,1
%A A220817 _N. J. A. Sloane_, Dec 22 2012
%E A220817 Terms a(11) and beyond from _Andrew Howroyd_, Sep 23 2018