Number of achiral triangular polyominoes with 6n cells and sixfold rotational symmetry.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =4a(6) =7a(7) =9a(8) =16a(9) =22a(10) =46a(11) =63a(12) =121a(13) =167a(14) =455a(15) =912a(16) =1263a(17) =2535a(18) =3514a(19) =7099a(20) =9873a(21) =20043a(22) =27956a(23) =56807a(24) =79397a(25) =161736a(26) =226559a(27) =462482a(28) =649100a(29) =1327165