a(n) = (2*n+2)!! - (2*n+1)!!.

Terms

a(0) =1a(1) =5a(2) =33a(3) =279a(4) =2895a(5) =35685a(6) =509985a(7) =8294895a(8) =151335135a(9) =3061162125a(10) =68000295825