Number of unoriented polyomino rings of length 2n with twofold rotational symmetry.

Terms

a(0) =0a(1) =1a(2) =0a(3) =1a(4) =1a(5) =3a(6) =3a(7) =9a(8) =13a(9) =35a(10) =59a(11) =147a(12) =280a(13) =669a(14) =1347a(15) =3142a(16) =6545a(17) =15110a(18) =32057a(19) =73625a(20) =158056a(21) =362280a(22) =783800a(23) =1795134a(24) =3906573a(25) =8946154a(26) =19558340