Number of polyominoes with n cells, symmetric about two orthogonal axes.

Terms

a(0) =0a(1) =1a(2) =1a(3) =1a(4) =1a(5) =2a(6) =3a(7) =4a(8) =4a(9) =8a(10) =10a(11) =15a(12) =17a(13) =30a(14) =35a(15) =60a(16) =64a(17) =117a(18) =128a(19) =236a(20) =241a(21) =459a(22) =476a(23) =937a(24) =912a(25) =1813a(26) =1789a(27) =3706a(28) =3456a(29) =7187