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%I #2 Apr 30 2014 01:30:13
%S 2,8,44,240,1248,6224,30052,141536,653744,2973072,13351192,59333408,
%T 261378944,1142885664,4965331028,21452209600,92230169232,394819386896,
%U 1683663903016,7155148746912,30313584232640,128067598573408
%N Number of polygons on n vertices with four faces such that the source of the polygon lies on exactly two faces.
%H T. Mansour and S. Severini, <a href="http://arXiv.org/abs/math.CO/0603225">Grid polygons from permutations and their enumeration by the kernel method</a>
%F a(n) = 2(n+2) * 4^(n-3) - 2*(n+1) * C(2*n-4,n-3).
%K nonn
%O 1,1
%A _Ralf Stephan_, May 08 2007
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