a(n) = C(3n,n) - C(2n,n).

Terms

a(0) =0a(1) =1a(2) =9a(3) =64a(4) =425a(5) =2751a(6) =17640a(7) =112848a(8) =722601a(9) =4638205a(10) =29860259a(11) =192831288a(12) =1248973544a(13) =8112024844a(14) =52820112480a(15) =344712308064