Number of Barlow packings with group P6(bar)m2 that repeat after 2n layers.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =6a(5) =12a(6) =28a(7) =56a(8) =119a(9) =240a(10) =496a(11) =990a(12) =2016a(13) =4032a(14) =8121a(15) =16256a(16) =32640a(17) =65268a(18) =130816a(19) =261630a(20) =523747a(21) =1047552a(22) =2096128a(23) =4192200a(24) =8386554a(25) =16773120a(26) =33550216a(27) =67100670a(28) =134209536a(29) =268418820