OFFSET
1,3
EXAMPLE
The a(6) = 19 rooted trees:
(((((o)))))
((((oo))))
(((o(o))))
(((ooo)))
((o((o))))
((o(oo)))
(((o)(o)))
((oo(o)))
((oooo))
(o(((o))))
(o((oo)))
(o(o(o)))
(o(ooo))
((o)((o)))
(oo((o)))
(oo(oo))
(o(o)(o))
(ooo(o))
(ooooo)
MATHEMATICA
strut[n_]:=strut[n]=If[n===1, {{}}, Select[Join@@Function[c, Union[Sort/@Tuples[strut/@c]]]/@IntegerPartitions[n-1], Or[Length[Union[#]]==1, Intersection@@#=={}]&]];
Table[Length[strut[n]], {n, 15}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Jul 05 2018
STATUS
approved