Floor(Gamma(2n/3)).

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =2a(5) =6a(6) =14a(7) =40a(8) =120a(9) =389a(10) =1355a(11) =5040a(12) =19884a(13) =82834a(14) =362880a(15) =1665837a(16) =7988887a(17) =39916800a(18) =207304278a(19) =1116669003a(20) =6227020800a(21) =35886673906a(22) =213407853922