OFFSET
0,2
LINKS
Gheorghe Coserea, Table of n, a(n) for n = 0..200
Luca G. Molinari, Nicola Manini, Enumeration of many-body skeleton diagrams, arXiv:cond-mat/0512342 [cond-mat.str-el], 2006.
FORMULA
a(n) = Sum_{k=0..n} A286796(n,k).
a(n) ~ 2^(n + 5/2) * n^(n+2) / exp(n+2). - Vaclav Kotesovec, Mar 08 2022
MATHEMATICA
max = 20; y0[x_, t_] = 1; y1[x_, t_] = 0; For[n = 1, n <= max, n++, y1[x_, t_] = (1 + x*(1 + 2*t + x*t^2)*y0[x, t]^2 + t*(1 - t)*x^2*y0[x, t]^3 + 2*x^2*y0[x, t]*D[y0[x, t], x])/(1 + 2*x*t) + O[x]^n // Normal // Simplify; y0[x_, t_] = y1[x, t]];
a[n_] := CoefficientList[SeriesCoefficient[y0[x, t]/(1 - x*t*y0[x, t]), {x, 0, n}], t] // Total;
Table[a[n], {n, 0, max-1}] (* Jean-François Alcover, May 24 2017, adapted from PARI *)
PROG
(PARI)
A286795_ser(N, t='t) = {
my(x='x+O('x^N), y0=1, y1=0, n=1);
while(n++,
y1 = (1 + x*(1 + 2*t + x*t^2)*y0^2 + t*(1-t)*x^2*y0^3 + 2*x^2*y0*y0');
y1 = y1 / (1+2*x*t); if (y1 == y0, break()); y0 = y1; ); y0;
};
Vec(A286796_ser(20, 1))
(PARI)
my(s=Ser(concat(1, vector(N+1, n, (2*n)!/(2^n*n!)))), g=(1/s - 1/s^2)/x);
1 - 1/subst(g, 'x, serreverse(x*g^2*s^2));
};
Vec(A286797_ser(20))
CROSSREFS
KEYWORD
nonn
AUTHOR
Gheorghe Coserea, May 21 2017
STATUS
approved