Number of Barlow packings with group P3(bar)m1(O) that repeat after 2n layers.

Terms

a(0) =2a(1) =3a(2) =8a(3) =16a(4) =40a(5) =73a(6) =165a(7) =320a(8) =666a(9) =1323a(10) =2709a(11) =5364a(12) =10880a(13) =21675a(14) =43577a(15) =87040a(16) =174590a(17) =348787a(18) =698709a(19) =1396736a(20) =2795400a(21) =5589672a(22) =11183445a(23) =22363920a(24) =44736510a(25) =89467563a(26) =178951005a(27) =357892096a(28) =715816960a(29) =1431611083