2,2

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

a(n) = 4(n-2)/(n-1) * C(2n-4,n-2). For n>2, a(n) = 4*A001791(n-2).

Sequence in context: A089883 A089932 A120926 this_sequence A072335 A081161 A032106

Adjacent sequences: A128647 A128648 A128649 this_sequence A128651 A128652 A128653

nonn

Ralf Stephan, May 08 2007