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 A058520 #4 Mar 30 2012 16:48:55
%S A058520 1,1,2,4,8,17,35,73,152,316,659,1376,2875,6004,12535,26173,54654,
%T A058520 114134,238348,497743,1039432,2170634,4532949,9466295,19768943,
%U A058520 41284768,86218046,180056039,376025957,785286555,1639979712,3424906290
%N A058520 Sequences of B-trees of order 3 with n labeled leaves.
%p A058520 spec := [C, {B=Union(Z, Subst(M, B)), M=Union(Prod(Z,Z),Prod(Z,Z,Z)), C=Sequence(B)}]; [seq(combstruct[count](spec, size=n), n=0..40)];
%Y A058520 Cf. A058518, A058519, A014535.
%K A058520 nonn
%O A058520 0,3
%A A058520 _N. J. A. Sloane_, Dec 21 2000