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

Terms

a(0) =1a(1) =5a(2) =31a(3) =200a(4) =1311a(5) =8665a(6) =57556a(7) =383556a(8) =2561871a(9) =17140007a(10) =114819351a(11) =769925568a(12) =5166845124a(13) =34696155564a(14) =233113911208