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

Terms

a(0) =1a(1) =2a(2) =12a(3) =96a(4) =880a(5) =8736a(6) =91392a(7) =992256a(8) =11075328a(9) =126297600a(10) =1465052160a(11) =17233182720a(12) =205074874368