OFFSET
1,2
LINKS
Gheorghe Coserea, Table of n, a(n) for n = 1..123
E. A. Bender, Z. Gao and N. C. Wormald, The number of labeled 2-connected planar graphs, Electron. J. Combin., 9 (2002), #R43.
P. S. Kolesnikov and B. K. Sartayev, On the special identities of Gelfand--Dorfman algebras, arXiv:2105.13815 [math.RA], 2021.
PROG
(PARI)
Q(n, k) = { \\ c-nets with n-edges, k-vertices
if (k < 2+(n+2)\3 || k > 2*n\3, return(0));
sum(i=2, k, sum(j=k, n, (-1)^((i+j+1-k)%2)*binomial(i+j-k, i)*i*(i-1)/2*
(binomial(2*n-2*k+2, k-i)*binomial(2*k-2, n-j) -
4*binomial(2*n-2*k+1, k-i-1)*binomial(2*k-3, n-j-1))));
};
seq(N) = {
my(x='x+O('x^(N+3)), t='t,
q=t*x*Ser(vector(N, n, Polrev(vector(2*n\3, k, Q(n, k)), t))),
d=serreverse((1+x)/exp(q/(2*t^2*x) + t*x^2/(1+t*x))-1),
g2=intformal(t^2/2*((1+d)/(1+x)-1)),
b=t*'x^2/2 + 'x*Ser(vector(N+1, n, subst(polcoeff(g2, n, 't), 'x, 't))),
g1=intformal(serreverse('x/exp(b'))/'x),
e1='x*Ser(vector(N, n, subst(polcoeff(serlaplace(g1), n, 't), 'x, 't))));
Vec(subst(e1, 't, 1));
};
seq(20)
CROSSREFS
KEYWORD
nonn
AUTHOR
Gheorghe Coserea, Sep 10 2017
STATUS
approved