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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =8a(6) =16a(7) =33a(8) =67a(9) =141a(10) =295a(11) =630a(12) =1340a(13) =2895a(14) =6237a(15) =13596a(16) =29556a(17) =64846a(18) =141976a(19) =313131a(20) =689425a(21) =1527099a(22) =3377723a(23) =7508724a(24) =16671776a(25) =37175536a(26) =82809462a(27) =185141322a(28) =413555554a(29) =926743719