Cluster series for honeycomb.

Terms

a(0) =1a(1) =3a(2) =6a(3) =12a(4) =24a(5) =33a(6) =60a(7) =99a(8) =156a(9) =276a(10) =438a(11) =597a(12) =1134a(13) =1404a(14) =2904a(15) =3522a(16) =6876a(17) =7548a(18) =16680a(19) =18153a(20) =39846a(21) =41805