Number of strictly 2-dimensional fixed polyominoes with n cells.

Terms

a(0) =0a(1) =0a(2) =4a(3) =17a(4) =61a(5) =214a(6) =758a(7) =2723a(8) =9908a(9) =36444a(10) =135266a(11) =505859a(12) =1903888a(13) =7204872a(14) =27394664a(15) =104592935a(16) =400795842a(17) =1540820540a(18) =5940738674a(19) =22964779658a(20) =88983512781a(21) =345532572676