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

Terms

a(0) =0a(1) =2a(2) =11a(3) =59a(4) =359a(5) =2519a(6) =20159a(7) =181439a(8) =1814399a(9) =19958399a(10) =239500799a(11) =3113510399a(12) =43589145599a(13) =653837183999