a(n) = n! / floor(2n/3)!.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =12a(5) =20a(6) =30a(7) =210a(8) =336a(9) =504a(10) =5040a(11) =7920a(12) =11880a(13) =154440a(14) =240240a(15) =360360a(16) =5765760a(17) =8910720a(18) =13366080a(19) =253955520a(20) =390700800a(21) =586051200a(22) =12893126400a(23) =19769460480a(24) =29654190720a(25) =741354768000