OFFSET
1,2
COMMENTS
A rooted tree is series-reduced if every non-leaf node has at least two branches.
FORMULA
EXAMPLE
The a(2) = 12 trees are (1(1(22))), (1(2(12))), (1(122)), (2(1(12))), (2(2(11))), (2(112)), ((11)(22)), ((12)(12)), (11(22)), (12(12)), (22(11)), (1122).
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]]]];
gro[m_]:=If[Length[m]==1, m, Union[Sort/@Join@@(Tuples[gro/@#]&/@Select[mps[m], Length[#]>1&])]];
Table[Length[gro[Ceiling[Range[1/2, n, 1/2]]]], {n, 4}]
PROG
(PARI) \\ See links in A339645 for combinatorial species functions.
cycleIndexSeries(n)={my(v=vector(2*n), vars=vector(2*n-2, i, sv(2+i))); v[1]=sv(1); for(n=2, #v, v[n] = substvec(polcoef( sExp(x*Ser(v[1..n])), n ), vars[1..n-2], vector(n-2))); sCartProd(x*Ser(v), 1/(1-x^2*symGroupCycleIndex(2)) + O(x*x^(2*n)))}
seq(n)={my(p=substvec(cycleIndexSeries(n), [sv(1), sv(2)], [1, 1])); vector(n, n, polcoef(p, 2*n))} \\ Andrew Howroyd, Jan 02 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jul 17 2018
EXTENSIONS
Terms a(6) and beyond from Andrew Howroyd, Jan 02 2021
STATUS
approved