Number of strictly 2-dimensional polyominoes with n cells.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =11a(5) =34a(6) =107a(7) =368a(8) =1284a(9) =4654a(10) =17072a(11) =63599a(12) =238590a(13) =901970a(14) =3426575a(15) =13079254a(16) =50107908a(17) =192622051a(18) =742624231a(19) =2870671949a(20) =11123060677a(21) =43191857687a(22) =168047007727a(23) =654999700402