a(n) = binomial(3n-1, n-1).

Terms

a(0) =0a(1) =1a(2) =5a(3) =28a(4) =165a(5) =1001a(6) =6188a(7) =38760a(8) =245157a(9) =1562275a(10) =10015005a(11) =64512240a(12) =417225900a(13) =2707475148a(14) =17620076360a(15) =114955808528a(16) =751616304549