OFFSET
1,2
FORMULA
a(n) = binomial(n^2,n)-2*binomial((n-1)*n,n)+binomial((n-1)^2,n).
EXAMPLE
a(2)=4: the two rotations of the (connected) domino and the two rotations of the disconnected domino consisting of two squares connected at a vertex.
MATHEMATICA
Table[Binomial[n^2, n]-2*Binomial[(n-1)n, n]+Binomial[(n-1)^2, n], {n, 20}] (* Harvey P. Dale, Oct 01 2013 *)
PROG
(PARI) a(n) = binomial(n^2, n) - 2*binomial((n-1)*n, n) + binomial((n-1)^2, n); \\ Michel Marcus, Aug 30 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
David Bevan, Jul 27 2009
STATUS
approved