a(n) = ceiling(Gamma(3n/4)).

Terms

a(0) =2a(1) =1a(2) =2a(3) =2a(4) =5a(5) =12a(6) =36a(7) =120a(8) =454a(9) =1872a(10) =8377a(11) =40320a(12) =207359a(13) =1133279a(14) =6552135a(15) =39916800a(16) =255371836a(17) =1710542069a(18) =11964299313a(19) =87178291200a(20) =660355655454