The Gauss factorial n_3!.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =8a(5) =40a(6) =40a(7) =280a(8) =2240a(9) =2240a(10) =22400a(11) =246400a(12) =246400a(13) =3203200a(14) =44844800a(15) =44844800a(16) =717516800a(17) =12197785600a(18) =12197785600a(19) =231757926400