OFFSET
0,4
LINKS
Evgeniy Krasko, Alexander Omelchenko, Enumeration of r-regular maps on the torus. Part I: Rooted maps on the torus, the projective plane and the Klein bottle. Sensed maps on the torus, Discrete Mathematics (2019) Vol. 342, Issue 2, 584-599. Also arXiv:1709.03225 [math.CO]. See last line of Section 2.
FORMULA
(-n+1)*a(n) +6*(4*n-5)*a(n-1) +72*(-2*n+3)*a(n-2)=0. - R. J. Mathar, Jan 17 2019
a(n) = (2^(2*n-3)*3^(n-2)*((2*n-1)!!/2^(n-1) - (n-1)!))/(n-1)!, n>0. - Jean-François Alcover, Feb 14 2019
MATHEMATICA
a[n_] := (2^(2n-3) 3^(n-2) ((2n-1)!!/2^(n-1) - (n-1)!))/(n-1)!; a[0] = 0;
Table[a[n], {n, 0, 21}] (* Jean-François Alcover, Feb 14 2019 *)
CoefficientList[Series[x/6 (1/(1-12x)^(3/2)-1/(1-12x)), {x, 0, 30}], x] (* Harvey P. Dale, Jun 18 2021 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jan 11 2019
STATUS
approved