OFFSET
1,2
COMMENTS
A rooted tree is series-reduced if every non-leaf node has at least two branches.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..200
EXAMPLE
The a(1) = 1 through a(4) = 13 trees:
(1) (2) (3) (4)
((1)(1)) (21) (31)
((1)(2)) ((1)(3))
((1)(1)(1)) ((2)(2))
((1)((1)(1))) ((1)(21))
((1)(1)(2))
((1)((1)(2)))
((2)((1)(1)))
((1)(1)(1)(1))
((1)((1)(1)(1)))
((1)(1)((1)(1)))
(((1)(1))((1)(1)))
((1)((1)((1)(1))))
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]]]];
sot[m_]:=If[UnsameQ@@m, Prepend[#, m], #]&[Join@@Table[Union[Sort/@Tuples[sot/@p]], {p, Select[mps[m], Length[#]>1&]}]];
Table[Length[Join@@Table[sot[m], {m, IntegerPartitions[n]}]], {n, 10}]
PROG
(PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, 1/n))))-1, -#v)}
seq(n)={my(p=prod(k=1, n, 1 + x^k + O(x*x^n)), v=vector(n)); for(n=1, n, v[n]=polcoef(p, n) + EulerT(v[1..n])[n]); v} \\ Andrew Howroyd, Oct 25 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Oct 07 2018
EXTENSIONS
Terms a(11) and beyond from Andrew Howroyd, Oct 25 2018
STATUS
approved