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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =1a(7) =3a(8) =9a(9) =21a(10) =51a(11) =117a(12) =262a(13) =598a(14) =1307a(15) =2987a(16) =6456a(17) =14752a(18) =31859a(19) =72839a(20) =157611a(21) =360472a(22) =782802a(23) =1791140a(24) =3904323a(25) =8936996a(26) =19553272