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

Terms

a(0) =1a(1) =1a(2) =5a(3) =7a(4) =33a(5) =57a(6) =279a(7) =561a(8) =2895a(9) =6555a(10) =35685a(11) =89055a(12) =509985a(13) =1381905a(14) =8294895a(15) =24137505a(16) =151335135a(17) =468934515a(18) =3061162125a(19) =10033419375a(20) =68000295825a(21) =234484536825