a(n) = Sum_{k=0..n-1} C(3*k,k)*C(3*n-3*k-2,n-k-1).

Terms

a(0) =0a(1) =1a(2) =7a(3) =48a(4) =327a(5) =2221a(6) =15060a(7) =102012a(8) =690519a(9) =4671819a(10) =31596447a(11) =213633696a(12) =1444131108a(13) =9760401756a(14) =65957919496a(15) =445671648228