OFFSET
1,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..200
EXAMPLE
The a(3) = 21 rooted trees are:
(1(11)), (1(1(1))), (1(12)), (1(22)), (1(1(2))), (1(2(1))), (1(2(2))), (2(12)), (2(11)), (2(2(1))), (2(1(2))), (2(1(1))), (1(23)), (1(2(3))), (1(3(2))), (2(13)), (2(1(3))), (2(3(1))), (3(12)), (3(1(2))), (3(2(1))).
MAPLE
b:= proc(n, k) option remember; `if`(n<2, k*n, (add(add(b(d, k)*
d, d=numtheory[divisors](j))*b(n-j, k), j=1..n-1))/(n-1))
end:
a:= n-> add(add(b(n, k-j)*binomial(k, j)*(-1)^j, j=0..k), k=0..n):
seq(a(n), n=1..21); # Alois P. Heinz, Dec 11 2020
MATHEMATICA
b[n_, k_] := b[n, k] = If[n<2, k*n, (Sum[Sum[b[d, k]*d, {d, Divisors[j]}]* b[n - j, k], {j, 1, n - 1}])/(n - 1)];
a[n_] := Sum[Sum[b[n, k - j]*Binomial[k, j]*(-1)^j, {j, 0, k}], {k, 0, n}];
Array[a, 21] (* Jean-François Alcover, Jan 04 2021, after Alois P. Heinz *)
PROG
(PARI) \\ See A141610 for U(n, k).
seq(n)={sum(k=1, n, U(n, k)*sum(r=k, n, binomial(r, k)*(-1)^(r-k)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Dec 11 2020
STATUS
approved