OFFSET
0,2
COMMENTS
Also: number of nonseparable planar rooted maps with n edges. - R. J. Mathar, Aug 18 2006
LINKS
T. R. S. Walsh, A. B. Lehman, Counting rooted maps by genus. III: Nonseparable maps, J. Combinatorial Theory Ser. B 18 (1975), 222-259.
FORMULA
a(n) = 2*A006013(n-1) = 2*(3n-2)!/(n!*(2*n-1)!). - R. J. Mathar, Aug 18 2006
G.f.: 1+2*g where g*(1-g)^2 = x. - Mark van Hoeij, Nov 10 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved