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 A058521 #11 Mar 29 2013 12:50:31
%S A058521 1,1,1,2,3,4,7,11,20,36,67,121,215,377,657,1154,2045,3666,6628,12063,
%T A058521 22079,40642,75264,140191,262457,493297,929703,1754941,3314509,
%U A058521 6258052,11803995,22232306,41801393,78453563,146987053,274957984
%N A058521 B-trees of order 5 with n labeled leaves.
%H A058521 <a href="/index/Ro#rooted">Index entries for sequences related to rooted trees</a>
%F A058521 G.f. A(x) satisfies: A(x) = x + A(x^2+x^3+x^4+x^5). [_Geoffrey Critzer_, Mar 28 2013]
%p A058521 spec := [ B, {B=Union(Z, Subst(M, B)), M=Union(Prod(Z,Z),Prod(Z,Z,Z),Prod(Z$4),Prod(Z$5))} ]: [seq(combstruct[count](spec, size=n), n=1..40)];
%t A058521 nn=38;f[x_]:=Sum[a[n]x^n,{n,0,nn}];a[0]=0;sol=SolveAlways[0==Series[f[x]-x-f[x^2+x^3+x^4+x^5],{x,0,nn}],x];Table[a[n],{n,0,nn}]/.sol (* _Geoffrey Critzer_, Mar 28 2013 *)
%Y A058521 Cf. A014535, A037026.
%K A058521 nonn
%O A058521 1,4
%A A058521 _N. J. A. Sloane_, Dec 21 2000