OFFSET
1,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..500
EXAMPLE
a(3) = 10 includes 5 trees and their color complements:
(1(12)), (1(22)), (1(1(2))), (1(2(1))), (1(2(2))).
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-> b(n, 2)-2*b(n, 1):
seq(a(n), n=1..25); # 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_] := b[n, 2] - 2*b[n, 1];
Array[a, 25] (* Jean-François Alcover, Jan 04 2021, after Alois P. Heinz *)
PROG
(PARI) \\ See A141610 for U(N, m)
seq(n)={U(n, 2) - 2*U(n, 1)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Dec 11 2020
STATUS
approved