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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =2a(5) =4a(6) =4a(7) =9a(8) =9a(9) =19a(10) =19a(11) =42a(12) =42a(13) =91a(14) =91a(15) =204a(16) =204a(17) =448a(18) =448a(19) =1007a(20) =1007a(21) =2233a(22) =2233a(23) =5034a(24) =5034a(25) =11242a(26) =11242a(27) =25400a(28) =25400a(29) =57033