a(n) = 2^n * (n + 3)!!.

Terms

a(0) =3a(1) =16a(2) =60a(3) =384a(4) =1680a(5) =12288a(6) =60480a(7) =491520a(8) =2661120a(9) =23592960a(10) =138378240a(11) =1321205760a(12) =8302694400a(13) =84557168640a(14) =564583219200