Arises from color-symmetrized counting of tensor invariants.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =27a(5) =103a(6) =391a(7) =1383a(8) =6260a(9) =32704a(10) =149045a(11) =812696a(12) =5034682a(13) =27895385a(14) =173642918a(15) =1221049609a(16) =7859267162a(17) =54735567331a(18) =426299653269