a(n) is the number of free polyiamonds of n cells with chessboard coloring.

Terms

a(0) =2a(1) =1a(2) =2a(3) =4a(4) =8a(5) =19a(6) =48a(7) =120a(8) =320a(9) =864a(10) =2372a(11) =6581a(12) =18470a(13) =52094a(14) =147966a(15) =421931a(16) =1208214a(17) =3470789a(18) =10001186a(19) =28892674a(20) =83671476a(21) =242823392a(22) =706090582a(23) =2056861981a(24) =6001601254a(25) =17538308071a(26) =51323923796a(27) =150389970179a(28) =441211039118