Sizes of successive shells in hexagonal (or A_2) lattice.

Terms

a(0) =1a(1) =6a(2) =6a(3) =6a(4) =12a(5) =6a(6) =6a(7) =12a(8) =6a(9) =12a(10) =12a(11) =6a(12) =6a(13) =12a(14) =12a(15) =6a(16) =12a(17) =12a(18) =12a(19) =6a(20) =18a(21) =12a(22) =12a(23) =12a(24) =12a(25) =6a(26) =12a(27) =12a(28) =6a(29) =12