OFFSET
2,1
LINKS
Andrew Howroyd, Table of n, a(n) for n = 2..200
W. G. Brown, Enumeration of non-separable planar maps, Canad. J. Math., 15 (1963), 526-545.
FORMULA
a(n) = 4*(7*n-11)*(3*n-5)!/((n-2)!*(2*n-1)!). - Emeric Deutsch, Mar 03 2004
G.f.: 2*(g+1)/(1-g)^3 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
Terms a(23) and beyond from Andrew Howroyd, Mar 29 2021
STATUS
approved