OFFSET
0,2
COMMENTS
There is a closed formula.
LINKS
Z. C. Gao, V. A. Liskovets and N. C. Wormald, Enumeration of unrooted odd-valent regular planar maps, Preprint, 2005.
MATHEMATICA
a[n_] := With[{b = Binomial[2n, n]}, (1/120)(36n-1) n^2 b^6 + (3n^2 b^3)/4 + n b^2/3 + (4/5) Boole[Divisible[2n+1, 5]] Binomial[2 Floor[n/5], Floor[n/5]] b];
a /@ Range[0, 9] (* Jean-François Alcover, Sep 01 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Valery A. Liskovets, Oct 10 2005
STATUS
approved