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A046649
a(n) is the number of nonseparable planar maps with 2*n+1 edges and a fixed outer face of 4 edges which are invariant under a rotation of a 1/2 turn. (Column 2 of A091665.)
2
2, 8, 34, 160, 806, 4256, 23256, 130416, 746350, 4341480, 25594530, 152585472, 918324904, 5572034240, 34048494608, 209347674768, 1294227005694, 8040125464280, 50165404177350, 314229490307040, 1975283452131990, 12456968750889600, 78790615438385760, 499700263517332800
OFFSET
2,1
LINKS
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
Column 2 of A091665.
Sequence in context: A084207 A151305 A150901 * A174808 A109972 A019023
KEYWORD
nonn,easy
EXTENSIONS
More terms from Emeric Deutsch, Mar 03 2004
Terms a(23) and beyond from Andrew Howroyd, Mar 29 2021
STATUS
approved