Floor((n-1)!/[n(n+1)]).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =12a(7) =70a(8) =448a(9) =3298a(10) =27490a(11) =255876a(12) =2631876a(13) =29652480a(14) =363242880a(15) =4807626352a(16) =68375130352