Number of unoriented polyomino rings of length 4n with fourfold rotational symmetry.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =6a(6) =10a(7) =21a(8) =38a(9) =80a(10) =157a(11) =336a(12) =691a(13) =1493a(14) =3164a(15) =6900a(16) =14880a(17) =32647a(18) =71212a(19) =157069a(20) =345216a(21) =764666a(22) =1689978a(23) =3756879a(24) =8338405a(25) =18593389a(26) =41410352a(27) =92583361a(28) =206790477a(29) =463400376