Number of axially symmetric polyominoes with n cells.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =6a(6) =9a(7) =23a(8) =38a(9) =90a(10) =147a(11) =341a(12) =564a(13) =1294a(14) =2148a(15) =4896a(16) =8195a(17) =18612a(18) =31349a(19) =70983a(20) =120357a(21) =271921a(22) =463712a(23) =1045559a(24) =1792582a(25) =4034832a(26) =6950579a(27) =15619507a(28) =27023509a(29) =60638559