OFFSET
2,2
LINKS
W. G. Brown, Enumeration of non-separable planar maps, Canad. J. Math., 15 (1963), 526-545.
W. G. Brown, Enumeration of non-separable planar maps [Annotated scanned copy]
FORMULA
a(2)=1; a(n) = 4*(7*n-15)*(3*n-6)!/((n-2)!*(2n-2)!). - Emeric Deutsch, Mar 03 2004
G.f.: (g+1)*(3*g+1)/(g-1)^2 where g*(1-g)^2 = x. - Mark van Hoeij, Nov 10 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
More terms from Emeric Deutsch, Mar 03 2004
STATUS
approved