Number of polyiamonds with n cells that tile the plane by 180-degree rotation (Conway criterion).

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =4a(5) =12a(6) =21a(7) =56a(8) =111a(9) =261a(10) =462a(11) =1431a(12) =1875a(13) =4696a(14) =9410a(15) =20099a(16) =24034a(17) =77884a(18) =79925a(19) =266786a(20) =358096a(21) =658594a(22) =796571a(23) =3227163a(24) =2980476a(25) =6268017a(26) =10167660a(27) =24321735a(28) =21477012a(29) =85964698