Number of prefix-closed polyominoes of area n.

Terms

a(0) =1a(1) =2a(2) =6a(3) =19a(4) =61a(5) =197a(6) =640a(7) =2091a(8) =6860a(9) =22566a(10) =74365a(11) =245397a(12) =810639a(13) =2680025a(14) =8865866