a(n) = n*binomial(n, n/2) if n is even otherwise 2^(n-1)*binomial(n-1, (n-1)/2).

Terms

a(0) =0a(1) =1a(2) =4a(3) =8a(4) =24a(5) =96a(6) =120a(7) =1280a(8) =560a(9) =17920a(10) =2520a(11) =258048a(12) =11088a(13) =3784704a(14) =48048a(15) =56229888a(16) =205920a(17) =843448320a(18) =875160a(19) =12745441280a(20) =3695120a(21) =193730707456a(22) =15519504a(24) =64899744a(26) =270415600a(28) =1123264800