a(n) = n*(n-1)*(n- 2)^2*2^(n-4)/3.

Terms

a(0) =0a(1) =0a(2) =1a(3) =16a(4) =120a(5) =640a(6) =2800a(7) =10752a(8) =37632a(9) =122880a(10) =380160a(11) =1126400a(12) =3221504a(13) =8945664a(14) =24227840a(15) =64225280a(16) =167116800a(17) =427819008a(18) =1079574528a(19) =2689597440a(20) =6624378880a(21) =16148070400a(22) =38997590016a(23) =93381984256a(24) =221878681600a(25) =523449139200