a(n) = abs(A021009(n, floor(n/2))).

Terms

a(0) =1a(1) =1a(2) =4a(3) =18a(4) =72a(5) =600a(6) =2400a(7) =29400a(8) =117600a(9) =1905120a(10) =7620480a(11) =153679680a(12) =614718720a(13) =14841066240a(14) =59364264960