a(n) = n * n!! - Sum_{k=1..n-1} k!! * a(n-k).

Terms

a(0) =1a(1) =3a(2) =4a(3) =19a(4) =31a(5) =168a(6) =323a(7) =1859a(8) =4072a(9) =24403a(10) =59423a(11) =368488a(12) =980123a(13) =6275139a(14) =17998264a(15) =118858755a(16) =364059999a(17) =2478263856a(18) =8045642683a(19) =56418223739a(20) =192980878976