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

Terms

a(0) =1a(1) =1a(2) =7a(3) =42a(4) =256a(5) =1586a(6) =9949a(7) =63004a(8) =401930a(9) =2579130a(10) =16628809a(11) =107636402a(12) =699030226a(13) =4552602248a(14) =29722279084a(15) =194458630304