OFFSET
1,6
COMMENTS
Also phylogenetic trees with no singleton leaves on integer partitions of n with no 1's.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..500
EXAMPLE
The a(4) = 1 through a(10) = 15 trees:
(22) (32) (33) (43) (44) (54) (55)
(42) (52) (53) (63) (64)
(222) (322) (62) (72) (73)
(332) (333) (82)
(422) (432) (433)
(2222) (522) (442)
((22)(22)) (3222) (532)
((22)(23)) (622)
(3322)
(4222)
(22222)
((22)(24))
((22)(33))
((23)(23))
((22)(222))
MATHEMATICA
sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];
mps[set_]:=Union[Sort[Sort/@(#/.x_Integer:>set[[x]])]&/@sps[Range[Length[set]]]];
pgtm[m_]:=Prepend[Join@@Table[Union[Sort/@Tuples[pgtm/@p]], {p, Select[mps[m], Length[#]>1&]}], m];
Table[Sum[Length[Select[pgtm[m], FreeQ[#, {_}]&]], {m, Select[IntegerPartitions[n], FreeQ[#, 1]&]}], {n, 10}]
PROG
(PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, 1/n))))-1, -#v)}
seq(n)={my(p=1/prod(k=2, n, 1 - x^k + O(x*x^n)), v=vector(n)); for(n=2, n, v[n]=polcoef(p, n) - 1 + EulerT(v[1..n])[n]); v} \\ Andrew Howroyd, Oct 25 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Oct 09 2018
EXTENSIONS
Terms a(16) and beyond from Andrew Howroyd, Oct 25 2018
STATUS
approved