a(n) = binomial(3*n+1, n+1).

Terms

a(0) =1a(1) =6a(2) =35a(3) =210a(4) =1287a(5) =8008a(6) =50388a(7) =319770a(8) =2042975a(9) =13123110a(10) =84672315a(11) =548354040a(12) =3562467300a(13) =23206929840a(14) =151532656696a(15) =991493848554