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

Terms

a(0) =1a(1) =4a(2) =16a(3) =78a(4) =456a(5) =3120a(6) =24480a(7) =216720a(8) =2136960a(9) =23224320a(10) =275788800a(11) =3552595200a(12) =49337164800a(13) =734788454400