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

Terms

a(0) =1a(1) =5a(2) =32a(3) =215a(4) =1469a(5) =10113a(6) =69892a(7) =484080a(8) =3357027a(9) =23297852a(10) =161757729a(11) =1123356551a(12) =7802235767a(13) =54191796523a(14) =376389282767