Number of Barlow packings with group P63mc that repeat after 2n layers.

Terms

a(0) =1a(1) =1a(2) =4a(3) =7a(4) =18a(5) =31a(6) =70a(7) =126a(8) =261a(9) =484a(10) =960a(11) =1800a(12) =3514a(13) =6643a(14) =12852a(15) =24457a(16) =47151a(17) =90157a(18) =173740a(19) =333498a(20) =643230a(21) =1238664a(22) =2392650a(23) =4620006a(24) =8939657a(25) =17302033a(26) =33538048a(27) =65042495a(28) =126289800a(29) =245361171