a(n) = Sum_{k=0..n} (-1)^k * binomial(3*k,n-k).

Terms

a(0) =1a(1) =-1a(2) =-2a(3) =2a(4) =6a(5) =-5a(6) =-17a(7) =12a(8) =48a(9) =-28a(10) =-135a(11) =63a(12) =378a(13) =-134a(14) =-1054a(15) =259a(16) =2927a(17) =-408a(18) =-8096a(19) =280a(20) =22305a(21) =1551a(22) =-61210a(23) =-10638a(24) =167310a(25) =46683a(26) =-455489a(27) =-175852a(28) =1234960a(29) =612380