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

Terms

a(0) =1a(1) =2a(2) =15a(3) =56a(4) =495a(5) =2736a(6) =29295a(7) =215424a(8) =2735775a(9) =25171200a(10) =370945575a(11) =4103654400a(12) =68916822975a(13) =891494553600