OFFSET
1,3
COMMENTS
There is an easy formula.
LINKS
V. A. Liskovets and T. R. S. Walsh, Enumeration of Eulerian and unicursal planar maps, Discr. Math., 282 (2004), 209-221.
MATHEMATICA
a[1] = a[2] = 1;
a[n_] := With[{m = Floor[(n+1)/2]}, 1/n 2^(n-3) Binomial[2n-2, n-1] + 2^(m-3) Binomial[2m-2, m-1]];
Array[a, 20] (* Jean-François Alcover, Aug 28 2019 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Valery A. Liskovets, Apr 07 2002
STATUS
approved