OFFSET
1,6
COMMENTS
The graphs also have n edges.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..200
Eric Weisstein's World of Mathematics, Unicyclic Graph
EXAMPLE
a(5)=1, a 4-cycle plus a pendant edge.
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)={concat([0, 0, 0], if(n<4, [], my(t=TreeGf(n-2)); my(g(e)=subst(t + O(x*x^(n\e)), x, x^e) + O(x*x^n)); Vec(sum(k=2, n\2, sumdiv(2*k, d, eulerphi(d)*g(d)^(2*k/d))/k + (g(1)^2+g(2))*g(2)^(k-1))/4)))} \\ Andrew Howroyd, May 22 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
David Bevan, Jul 24 2012
EXTENSIONS
Terms a(16) and beyond from Andrew Howroyd, May 22 2018
STATUS
approved