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, 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

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

Column 2 of A091665.

Sequence in context: A084207 A151305 A150901 * A174808 A109972 A019023

Adjacent sequences: A046646 A046647 A046648 * A046650 A046651 A046652

Keyword

nonn,easy

Author

N. J. A. Sloane

Extensions

More terms from Emeric Deutsch, Mar 03 2004

Terms a(23) and beyond from Andrew Howroyd, Mar 29 2021

Status

approved