1,1

Table of n, a(n) for n=1..22.

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

a(n) = 2(n+2) * 4^(n-3) - 2*(n+1) * C(2*n-4,n-3).

Sequence in context: A018985 A018986 A192469 * A208044 A291697 A047851

Adjacent sequences: A128653 A128654 A128655 * A128657 A128658 A128659

nonn

Ralf Stephan, May 08 2007

approved