a(n) = Sum_{k=0..n} binomial(3*n+2*k+1,n-k).

Terms

a(0) =1a(1) =5a(2) =31a(3) =201a(4) =1326a(5) =8824a(6) =59017a(7) =395986a(8) =2662707a(9) =17932178a(10) =120901309a(11) =815824636a(12) =5508658729a(13) =37214906598a(14) =251515412373