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 A037026 #22 Feb 16 2025 08:32:37
%S A037026 0,1,1,1,2,2,4,5,9,15,28,45,73,116,199,345,601,1021,1738,2987,5244,
%T A037026 9321,16720,30103,54345,97996,176011,314045,556577,980852,1722451,
%U A037026 3020214,5298570,9316456,16440262,29136489,51860214,92643425,165934156
%N A037026 Number of B-trees of order 4 with n leaves.
%H A037026 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/B-Tree.html">B-tree</a>
%H A037026 <a href="/index/Ro#rooted">Index entries for sequences related to rooted trees</a>
%F A037026 G.f. A(x) satisfies: A(x) = x + A(x^2+x^3+x^4). [_Geoffrey Critzer_, Mar 28 2013]
%p A037026 spec := [ B, {B=Union(Z, Subst(M, B)), M=Union(Prod(Z,Z),Prod(Z,Z,Z),Prod(Z$4))} ]: [ seq(combstruct[count](spec, size=n), n=0..42) ];
%t A037026 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,0,nn}],x];Table[a[n],{n,0,nn}]/.sol (* _Geoffrey Critzer_, Mar 28 2013 *)
%Y A037026 Cf. A014535.
%K A037026 nonn
%O A037026 0,5
%A A037026 _N. J. A. Sloane_, _Paul Zimmermann_