A128656 Number of polygons on n vertices with four faces such that the source of the polygon lies on exactly two faces.

2, 8, 44, 240, 1248, 6224, 30052, 141536, 653744, 2973072, 13351192, 59333408, 261378944, 1142885664, 4965331028, 21452209600, 92230169232, 394819386896, 1683663903016, 7155148746912, 30313584232640, 128067598573408

Offset

1,1

Links

Table of n, a(n) for n=1..22.

T. Mansour and S. Severini, Grid polygons from permutations and their enumeration by the kernel method

Formula

a(n) = 2(n+2) * 4^(n-3) - 2*(n+1) * C(2*n-4,n-3).

Crossrefs

Sequence in context: A018985 A018986 A192469 * A208044 A362413 A362408

Adjacent sequences: A128653 A128654 A128655 * A128657 A128658 A128659

Keyword

nonn

Author

Ralf Stephan, May 08 2007

Status

approved