Ceiling(Gamma(2n/3)).

Terms

a(0) =2a(1) =1a(2) =1a(3) =2a(4) =3a(5) =6a(6) =15a(7) =41a(8) =120a(9) =390a(10) =1356a(11) =5040a(12) =19885a(13) =82835a(14) =362880a(15) =1665838a(16) =7988888a(17) =39916800a(18) =207304279a(19) =1116669004a(20) =6227020800a(21) =35886673907a(22) =213407853923