Number of polyominoes of 2n-1 cells that span an n X n square.

Terms

a(0) =1a(1) =1a(2) =6a(3) =18a(4) =73a(5) =255a(6) =950a(7) =3473a(8) =13006a(9) =48840a(10) =185353a(11) =706404a(12) =2706608a(13) =10404625a(14) =40126430a(15) =155133811a(16) =601119492a(17) =2333671638a(18) =9075290555a(19) =35345525798a(20) =137847145330a(21) =538258922839