OFFSET
1,3
COMMENTS
A powerful uniform rooted tree with n nodes is either a single powerful uniform branch with n-1 nodes, or a powerful uniform multiset (all multiplicities are equal to the same number > 1) of powerful uniform rooted trees with a total of n-1 nodes.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..1000
Gus Wiseman, All 35 powerful uniform rooted trees with 11 nodes.
EXAMPLE
The a(8) = 12 powerful uniform rooted trees:
(((((((o)))))))
((((((oo))))))
(((((o)(o)))))
((((o))((o))))
(((((ooo)))))
(((o)(o)(o)))
((((oooo))))
(((oo)(oo)))
((oo(o)(o)))
(((ooooo)))
((oooooo))
(ooooooo)
MATHEMATICA
rurt[n_]:=If[n==1, {{}}, Join@@Table[Select[Union[Sort/@Tuples[rurt/@ptn]], Or[Length[#]==1, And[Min@@Length/@Split[#]>=2, SameQ@@Length/@Split[#]]]&], {ptn, IntegerPartitions[n-1]}]];
Table[Length[rurt[n]], {n, 15}]
PROG
(PARI) WeighT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, (-1)^(n-1)/n))))-1, -#v)}
seq(n)={my(v=vector(n)); v[1]=1; for(n=1, n-1, my(u=WeighT(v[1..n])); v[n+1] = sumdiv(n, d, u[d]) - u[n] + v[n]); v} \\ Andrew Howroyd, Dec 09 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 31 2018
EXTENSIONS
Terms a(21) and beyond from Andrew Howroyd, Dec 09 2020
STATUS
approved