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

Terms

a(0) =1a(1) =5a(2) =34a(3) =263a(4) =2159a(5) =18312a(6) =158536a(7) =1391999a(8) =12348587a(9) =110408669a(10) =993288215a(11) =8980902802a(12) =81537707501a(13) =742853687870