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A128656
Number of polygons on n vertices with four faces such that the source of the polygon lies on exactly two faces.
0
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
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
KEYWORD
nonn
AUTHOR
Ralf Stephan, May 08 2007
STATUS
approved