OFFSET
1,1
LINKS
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
KEYWORD
nonn
AUTHOR
Ralf Stephan, May 08 2007
STATUS
approved