a(n) = |Sum_{k=0..n} (-1)^k*(2k)!!|.

Terms

a(0) =1a(1) =1a(2) =7a(3) =41a(4) =343a(5) =3497a(6) =42583a(7) =602537a(8) =9719383a(9) =176075177a(10) =3539816023a(11) =78209790377