%I #11 Dec 23 2016 17:39:39

%S 1,4,48,956,26490,937342,40291608,2036155284,118202408622,

%T 7747410899954,565695467415936,45525704815717568,4002930269944724664,

%U 381750656962687053108,39244733577786624617904,4325973539461955182836900,508971415418900757219557142

%N Number of different fixed (possibly) disconnected n-ominoes bounded (not necessarily tightly) by an n*n square.

%F a(n) = binomial(n^2,n)-2*binomial((n-1)*n,n)+binomial((n-1)^2,n).

%e 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.

%t Table[Binomial[n^2,n]-2*Binomial[(n-1)n,n]+Binomial[(n-1)^2,n],{n,20}] (* _Harvey P. Dale_, Oct 01 2013 *)

%o (PARI) a(n) = binomial(n^2,n) - 2*binomial((n-1)*n,n) + binomial((n-1)^2,n); \\ _Michel Marcus_, Aug 30 2013

%Y Cf. A162673, A162674, A162675, A162677.

%K nonn

%O 1,2

%A _David Bevan_, Jul 27 2009