a(n) = binomial(3*n+2, n-1).

Terms

a(0) =1a(1) =8a(2) =55a(3) =364a(4) =2380a(5) =15504a(6) =100947a(7) =657800a(8) =4292145a(9) =28048800a(10) =183579396a(11) =1203322288a(12) =7898654920a(13) =51915526432a(14) =341643774795