OFFSET
2,2
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 2..200
T. Mansour and S. Severini, Grid polygons from permutations and their enumeration by the kernel method, arXiv:math/0603225 [math.CO], 2006.
FORMULA
a(n) = 4*(n-2)/(n-1) * C(2*n-4,n-2).
For n>2, a(n) = 4*A001791(n-2).
MATHEMATICA
Table[4 Binomial[2n-4, n-3], {n, 2, 25}] (* Jean-François Alcover, Dec 05 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, May 08 2007
STATUS
approved