Duplicate of A067819.

Terms

a(0) =3a(1) =12a(2) =42a(3) =144a(4) =728a(5) =2688a(6) =10080a(7) =39312a(8) =127008a(9) =423360a(10) =2419200a(11) =6773760a(12) =32140800