%I #4 Mar 30 2012 16:48:55
%S 1,1,2,4,8,17,35,73,152,316,659,1376,2875,6004,12535,26173,54654,
%T 114134,238348,497743,1039432,2170634,4532949,9466295,19768943,
%U 41284768,86218046,180056039,376025957,785286555,1639979712,3424906290
%N Sequences of B-trees of order 3 with n labeled leaves.
%p 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 Cf. A058518, A058519, A014535.
%K nonn
%O 0,3
%A _N. J. A. Sloane_, Dec 21 2000