Cluster series for hexagonal lattice.

Terms

a(0) =1a(1) =6a(2) =18a(3) =48a(4) =126a(5) =300a(6) =750a(7) =1686a(8) =4074a(9) =8868a(10) =20892a(11) =44634a(12) =103392a(13) =216348a(14) =499908a(15) =1017780a(16) =2383596a(17) =4648470a(18) =11271102a(19) =20763036a(20) =52671018a(21) =91377918