Theta series of planar hexagonal lattice A_2.

Terms

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