OFFSET
1,2
LINKS
Mireille Bousquet-Mélou, Andrew Elvey Price, Andrew Price, The generating function of planar Eulerian orientations, arXiv:1803.08265 [math.CO], 2018.
Mireille Bousquet-Mélou, Andrew Elvey Price, Paul Zinn-Justin, Eulerian orientations and the six-vertex model on planar map, arXiv:1902.07369 [math.CO], 2019. See Theorem 2.
MATHEMATICA
m = 20;
aa = Array[a, m];
R[t_] = aa.t^Range[m];
eq = Thread[CoefficientList[t-Sum[1/(n+1) Binomial[2n, n] Binomial[3n, n]* R[t]^(n+1), {n, 0, m}] + O[t]^(m+1), t] == 0];
aa /. Solve[eq, aa][[1]] (* Jean-François Alcover, Feb 25 2019 *)
PROG
(PARI) lista(nn) = {my(v = vector(nn), R, P, c, r, s); kill(y); for (n=1, nn, v[n] = y; R = sum(k=1, n, v[k]*t^k); P = sum(k=0, n, binomial(2*k, k)*binomial(3*k, k)/(k+1)*R^(k+1)); c = polcoef(P, n, t); r = polcoef(c, 0, y); s = polcoef(c, 1, y); if (n==1, v[n] = (1-r)/s, v[n] = -r/s); ); R = sum(k=1, #v, v[k]*t^k); vector(nn, k, polcoef(R, k, t)); }
CROSSREFS
KEYWORD
sign
AUTHOR
Michel Marcus, Feb 21 2019
STATUS
approved