OFFSET
0,4
COMMENTS
The graphs considered here can have loops but not parallel edges.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..500
EXAMPLE
Representatives of the a(3) = 3 graphs are:
{{1,2}, {1,3}, {2,3}},
{{1}, {1,2}, {1,3}},
{{1}, {1,2}, {2,3}}.
PROG
(PARI) \\ TreeGf gives gf of A000081.
TreeGf(N)={my(A=vector(N, j, 1)); for (n=1, N-1, A[n+1] = 1/n * sum(k=1, n, sumdiv(k, d, d*A[d]) * A[n-k+1] ) ); x*Ser(A)}
seq(n)={my(t=TreeGf(n)); my(g(e)=subst(t + O(x*x^(n\e)), x, x^e) + O(x*x^n)); Vec(1 + (sum(d=1, n, eulerphi(d)/d*log(1/(1-g(d)))) + ((1+g(1))^2/(1-g(2))-1)/2 - (g(1)^2 + g(2)))/2)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Jan 11 2024
STATUS
approved